Fighting vessel was going round river with it (7)
I believe the answer is:
warship
'fighting vessel' is the definition.
(vessel used in fighting)
'was going round river with it' is the wordplay.
'going round' indicates putting letters inside (some letters go around others).
'river' becomes 'r' (abbreviation for river).
'with it' becomes 'hip' (I've seen this in another clue).
'was' enclosing 'r' is 'wars'.
'wars'+'hip'='WARSHIP'
(Other definitions for warship that I've seen before include "Eg, destroyer, cruiser" , "frigate perhaps" , "Fighting vessel" , "Naval asset" , "Navy asset" .)