Grandfather taking sides -- that hurts, one complains (6)
I believe the answer is:
grouch
'one complains' is the definition.
(a grouch complains about things)
'grandfather taking sides that hurts' is the wordplay.
'taking sides' suggests removing the centre (only letters at the sides).
'that hurts' becomes 'ouch'.
'grandfather' with its middle removed is 'gr'.
'gr'+'ouch'='GROUCH'
(Other definitions for grouch that I've seen before include "one known for grumbling" , "Grumble - complain" , "'Crosspatch, grump (6)'" , "Bellyache" , "Grumbler - gripe" .)