Fish out a letter that's unfriendly (4)
I believe the answer is:
cold
'unfriendly' is the definition.
(similar in meaning)
'fish out a letter' is the wordplay.
'fish' becomes 'cod' (type of fish).
'out' indicates putting letters inside.
'a letter' becomes 'l' ().
'cod' going around 'l' is 'COLD'.
'that's' is the link.
(Other definitions for cold that I've seen before include "Unemotional; infection" , "Chill" , "Stand-offish" , "At low temperature" , "Chilly; minor ailment" .)