Light exercises taken in by sailor (5)
I believe the answer is:
taper
'light' is the definition.
(taper is a kind of light)
'exercises taken in by sailor' is the wordplay.
'exercises' becomes 'pe'.
'taken in by' is an insertion indicator.
'sailor' becomes 'tar' (tar is a kind of sailor).
'pe' put inside 'tar' is 'TAPER'.
(Other definitions for taper that I've seen before include "Narrow towards the end" , "Slender candle" , "person making recording" , "Narrow to a point" , "Thin wick" .)