The timekeeper not in so be careful (5,3)
I believe the answer is:
watch out
'be careful' is the definition.
I can't tell whether this defines the answer.
'the timekeeper not in' is the wordplay.
'the timekeeper' becomes 'watch' (watch is a kind of timekeeper).
'not in' becomes 'out' ('out' is the opposite to 'in').
'watch'+'out'='WATCH-OUT'
'so' is the link.
Can you help me to learn more?
(Other definitions for watch out that I've seen before include "this could make synchronisation difficult!" , "Be vigilant" , "Be careful!" .)