Fish out a letter that's unfriendly (4)
I believe the answer is:
cold
'unfriendly' is the definition.
('cold' can be similar in meaning to 'unfriendly')
'fish out a letter' is the wordplay.
'fish' becomes 'cod' (cod is a kind of fish).
'out' means one lot of letters goes inside another.
'a letter' becomes 'l' (**).
'cod' enclosing 'l' is 'COLD'.
'that's' acts as a link.
(Other definitions for cold that I've seen before include "Unemotional; infection" , "Chill" , "Stand-offish" , "At low temperature" , "Chilly; minor ailment" .)