Being alert I left in time (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' is an insertion indicator.
'time' becomes 'age' (an age is a period of time).
'i'+'l'='il'
'il' inserted into 'age' is 'AGILE'.
(Other definitions for agile that I've seen before include "Nimble, active" , "Mentally acute" , "Quick to move" , "athletic" , "Dexterous" .)