Group of relations mostly head for France (6)
I believe the answer is:
sextet
'group' is the definition.
(I know that sextet is a type of group)
'relations mostly head for france' is the wordplay.
'relations' becomes 'sex' (euphemism for sex).
'mostly' means to remove the last letter.
'head for france' becomes 'tete' (French for 'head').
'tete' with its last letter removed is 'tet'.
'sex'+'tet'='SEXTET'
'of' acts as a link.
(Other definitions for sextet that I've seen before include "Half-dozen in a group" , "Piece for six" , "A group of six singers" , "Musical group" , "Group of six performers" .)