Seamen not normally crossing south, as a whole (2,5)
I believe the answer is:
en masse
'as a whole' is the definition.
(thesaurus)
'seamen not normally crossing south' is the wordplay.
'not normally' indicates an anagram.
'crossing' means one lot of letters goes inside another.
'south' becomes 's' ('S' can be a synonym of 'south').
'seamen' with letters rearranged gives 'enmase'.
'enmase' placed around 's' is 'EN MASSE'.
(Other definitions for en masse that I've seen before include "All together (Fr.)" , "All together as a group" , "In force" , "as a bunch" , "As a body (Fr.)" .)