Reaching conclusion, ready to complain (5)
I believe the answer is:
gripe
'complain' is the definition.
(I know that gripe is a more specific form of the action complain)
'reaching conclusion ready' is the wordplay.
'conclusion' suggests the final letters.
'ready' becomes 'ripe' (similar in meaning).
The last letter of 'reaching' is 'g'.
'g'+'ripe'='GRIPE'
'to' acts as a link.
(Other definitions for gripe that I've seen before include "Grievance" , "carp" , "beef" , "Grumble (colloq.)" , "Complain, grouse" .)