Mother's possibly seen going round in the altogether! (2,5)
I believe the answer is:
en masse
'the altogether' is the definition.
(thesaurus)
'mother's possibly seen going round' is the wordplay.
'mother's' becomes 'mas' (Ma is another word for mother or mum).
'possibly' indicates anagramming the letters.
'going round' means one lot of letters goes inside another.
'seen' anagrammed gives 'ense'.
'mas' put into 'ense' is 'EN MASSE'.
'in' is the link.
(Other definitions for en masse that I've seen before include "in concert" , "In a group U altogether" , "Sees man (anag.)" , "In force" , "All together (Fr.)" .)