Complaints mature in case of grievances (6)
I believe the answer is:
gripes
'complaints' is the definition.
(I've seen this before)
'mature in case of grievances' is the wordplay.
'mature' becomes 'ripe' ('ripe' can be similar in meaning to 'mature').
'in' is an insertion indicator.
'case of' suggests removing the centre (outsides of).
'grievances' with its middle taken out is 'gs'.
'ripe' placed within 'gs' is 'GRIPES'.
(Other definitions for gripes that I've seen before include "Complaints" , "Grouses, beefs" .)