Men working on rough seas as a group (2,5)
I believe the answer is:
en masse
'as a group' is the definition.
(synonyms)
'men working on rough seas' is the wordplay.
'working' indicates anagramming the letters.
'on' is a charade indicator (letters next to each other).
'rough' indicates anagramming the letters (a rough or broken ordering of the letters).
'men' is an anagram of 'enm'.
'seas' anagrammed gives 'asse'.
'enm'+'asse'='EN MASSE'
(Other definitions for en masse that I've seen before include "As a body (Fr.)" , "Sees man (anag.)" , "All together - in France (2.5)" , "All together as a group" , "in concert" .)