Is it a cover for a love letter? (4)
I believe the answer is:
iota
'letter?' is the definition.
(iota is a kind of letter)
'is it a cover for a love' is the wordplay.
'cover for' means one lot of letters goes inside another.
'a love' becomes 'o' (In tennis, 'love' means 'zero').
'it'+'a'='ita'
'ita' going around 'o' is 'IOTA'.
(Other definitions for iota that I've seen before include "Tiny piece" , "Tiny or scarcely detectable amount" , "Tiniest scrap" , "Contrivance" , "Just a jot, like Greek letter" .)