In headwind left to become smaller (7)
I believe the answer is:
dwindle
'become smaller' is the definition.
(I've seen this before)
'in headwind left to' is the wordplay.
'in' indicates a hidden word.
'DWINDLE' can be found hidden inside 'headwind left to'.
(Other definitions for dwindle that I've seen before include "Get smaller, less" , "Diminish gradually" , "Become smaller or fewer" , "Grow less - waste away" , "Grow smaller, taper off" .)