I left in time being alert (5)
I believe the answer is:
agile
'being alert' is the definition.
(I've seen this before)
'i left in time' is the wordplay.
'left' becomes 'l' (common abbreviation).
'in' means one lot of letters goes inside another.
'time' becomes 'age' (an age is a period of time).
'i'+'l'='il'
'il' put into 'age' is 'AGILE'.
(Other definitions for agile that I've seen before include "Nimble, spry" , "Quick in movement" , "Nimble, quick-moving" , "Nimble, supple" , "Nifty" .)