Friend’s gone — I’m back inside (5)
I believe the answer is:
amigo
'friend's' is the definition.
('friend' in Spanish)
'gone i'm back inside' is the wordplay.
'gone' becomes 'ago' (eg 'two weeks gone' could mean 'two weeks ago').
'back' is a reversal indicator.
'inside' indicates putting letters inside.
'im' written backwards gives 'mi'.
'ago' enclosing 'mi' is 'AMIGO'.
(Other definitions for amigo that I've seen before include "Friend in Spanish-speaking area" , "Male (Spanish?) friend" , "Hispanic friend" , "buddy abroad?" , "in a foreign land, my friend?" .)