Sailor tackles exercise to get lighter (5)
I believe the answer is:
taper
'lighter' is the definition.
(I've seen this before)
'sailor tackles exercise' is the wordplay.
'sailor' becomes 'tar' (tar can mean a sailor).
'tackles' indicates putting letters inside (tackle can mean to intercept or surround).
'exercise' becomes 'pe' (Physical Education).
'tar' enclosing 'pe' is 'TAPER'.
'to get' is the link.
(Other definitions for taper that I've seen before include "Become thinner towards one end" , "Recorder" , "Gradually become thinner" , "Get more slender" , "Spill - become narrower towards one end" .)