Muslim fighter overwhelmed by happiness in pilgrimage site (7)
I believe the answer is:
galilee
'pilgrimage site' is the definition.
(region of Israel)
'muslim fighter overwhelmed by happiness' is the wordplay.
'muslim fighter' becomes 'ali' (Muhammad Ali).
'overwhelmed by' means one lot of letters goes inside another (I've seen this in other clues).
'happiness' becomes 'glee' (glee is a kind of happiness).
'ali' put into 'glee' is 'GALILEE'.
'in' is the link.
(Other definitions for galilee that I've seen before include "Freshwater sea of Israel" , "Holy land lake; church porch" , "Sea of . . . . . . . is a Biblical lake now in Israel" , "Holy land region" , "Biblical Sea west of the Jordan" .)