Unnecessary enmity on steamship (8)
I believe the answer is:
needless
'unnecessary' is the definition.
('needless' can be similar in meaning to 'unnecessary')
'enmity on steamship' is the wordplay.
'enmity' becomes 'needle' (I've seen this before).
'on' is a charade indicator (letters next to each other).
'steamship' becomes 'ss'.
'needle'+'ss'='NEEDLESS'
(Other definitions for needless that I've seen before include "Uncalled-for" , "Gratuitous" , "Lessened, being without good cause" , "Surplus to requirements" , "Otiose" .)