to keep a clear head: to stay unclouded and self-possessed, the way a person who has not been drinking still sees straight
In ordinary Greek this word meant literally not drunk: the condition of someone who has kept away from wine and whose head is clear. Every use in the New Testament is a picture drawn from that condition. None of the six is a rule about what to drink; each asks for a mind that is not fogged, not swept along, and not slow to see what is in front of it.
It is always a command, and it almost always keeps company with other commands. Paul sets it beside staying awake (γρηγορέω) in 1 Thessalonians 5:6, and Peter pairs the two again in 1 Peter 5:8; Peter also puts it next to good sense (σωφρονέω) in 1 Peter 4:7. The partners show what this word adds. Being awake is having your eyes open, and good sense is having a balanced mind. Sobriety is the clear judgment that makes either of them worth anything.
What the clear head is for changes with the setting. Where the coming day is in view, it is the bearing of people who know what time it is and refuse to drift into a stupor with everyone else. Where danger or hard work is in view, it is the refusal to panic, to lose your footing, or to let strain do your thinking for you. The demand is the same in both: stay unclouded, so you can see what is actually happening and act on it.