Complain heartless left has last word (6)
I believe the answer is:
lament
'complain' is the definition.
(I've seen this before)
'heartless left has last word' is the wordplay.
'heartless' suggests removing the centre.
'has' is an insertion indicator.
'last word' becomes 'amen' (I've seen this before).
'left' with its centre removed is 'lt'.
'lt' enclosing 'amen' is 'LAMENT'.
(Other definitions for lament that I've seen before include "Dart-throwing line" , "Express sorrow" , "Song of sorrow" , "Mental (anag)" , "Mourn, grieve for" .)