“A friend has a right,” I put in, “to be biased” (7)
I believe the answer is:
partial
'be biased' is the definition.
('partial' can be similar in meaning to 'biased')
'a friend has a right i put in' is the wordplay.
'a friend' becomes 'pal' (pal is a kind of friend**).
'has' is an insertion indicator.
'right' becomes 'rt'.
'put in' is an insertion indicator.
'a'+'rt'='art'
'pal' enclosing 'art' is 'partal'.
'partal' going around 'i' is 'PARTIAL'.
'to' is the link.
(Other definitions for partial that I've seen before include "Fond of" , "Not complete or total, maybe an eclipse" , "Keen" , "Showing favouritism" , "Not complete, or with strong liking for" .)