Humourless manipulation accepted by ship's mates (7)
I believe the answer is:
spouses
'mates' is the definition.
('spouse' can be a synonym of 'mate')
'humourless manipulation accepted by ship's' is the wordplay.
'humourless' becomes 'po' (I've seen this in other clues).
'manipulation' becomes 'use' (synonyms).
'accepted by' is an insertion indicator (inserted letters are accepted).
'ship' becomes 'SS' (prefix in ship names eg SS Great Britain).
'po'+'use'='pouse'
'pouse' put within 'ss' is 'SPOUSES'.
(Other definitions for spouses that I've seen before include "Wives or husbands" , "Partners in marriage" .)