Upon swell, exercising rowers – what each must do? (4,4,6)
I believe the answer is:
pull ones weight
'what each must do?' is the definition.
'upon swell exercising rowers' is the wordplay.
'exercising' is an anagram indicator (letters exercise or move around).
'rowers' becomes 'eight' (I've seen this before).
'upon'+'swell'='uponswell'
'uponswell' is an anagram of 'pullonesw'.
'pullonesw'+'eight'='PULL ONES WEIGHT'
(Other definitions for pull ones weight that I've seen before include "Don't skive" , "Join in effort" .)