Fish carried in steamship for parties (5)
I believe the answer is:
sides
'for parties' is the definition.
(eg the two parties in an argument)
'fish carried in steamship' is the wordplay.
'fish' becomes 'ide' (I've seen this before).
'carried in' means one lot of letters goes inside another.
'steamship' becomes 'ss' (nautical abbreviation).
'ide' going within 'ss' is 'SIDES'.
(Other definitions for sides that I've seen before include "Borders?" , "Faces" , "camps" , "Outer edges, left and right" , "Teams or flat surfaces" .)