Put, inside it, a love letter (4)
I believe the answer is:
iota
'letter' is the definition.
(Greek letter)
'put inside it a love' is the wordplay.
'put inside' is an insertion indicator.
'love' becomes 'o' (love means zero in tennis).
'it'+'a'='ita'
'ita' enclosing 'o' is 'IOTA'.
(Other definitions for iota that I've seen before include "Jot or tittle" , "Trace" , "Ninth letter of Greek alphabet, small amount" , "A little piece" , "Tiny bit" .)