'Relacher' primarily is French for 'relax' (4)
I believe the answer is:
rest
'relax' is the definition.
(I've seen this before)
'relacher primarily is french' is the wordplay.
'primarily' says to take the initial letters.
'is french' becomes 'est' ('is' in French).
The initial letter of 'relacher' is 'r'.
'r'+'est'='REST'
'for' acts as a link.
(Other definitions for rest that I've seen before include "Remainder; be inactive" , "Repose, others" , "Slumber" , "still holding it" , "'''God ... ye, merry gentlemen'' (carol) (4)'" .)