Complained taking exercise in the square (6)
I believe the answer is:
griped
'complained' is the definition.
(griping is a kind of complaining)
'exercise in the square' is the wordplay.
'exercise' becomes 'pe' (Physical Education).
'in' means one lot of letters goes inside another.
'the square' becomes 'grid' (a grid is a square layout).
'pe' placed inside 'grid' is 'GRIPED'.
'taking' is the link.
(Other definitions for griped that I've seen before include "Moaned constantly" , "Complained persistently" .)