Frank in love with writer (4)
I believe the answer is:
open
'frank' is the definition.
(synonyms)
'love with writer' is the wordplay.
'love' becomes 'o' (love means zero in tennis).
'with' says to put letters next to each other.
'writer' becomes 'pen' (something which writes).
'o'+'pen'='OPEN'
'in' is the link.
(Other definitions for open that I've seen before include "'Frank, not secret (4)'" , "Not secret or hidden" , "Sports contest" , "Not closed; set going" , "Not enclosed" .)