Using computers, adult pens love letter (4)
I believe the answer is:
iota
'letter' is the definition.
(Greek letter)
'using computers adult pens love' is the wordplay.
'using computers' becomes 'IT' (information technology).
'adult' becomes 'a' (abbreviation).
'pens' indicates putting letters inside (inserted letters are held or penned in).
'love' becomes 'o' (love means zero in tennis).
'it'+'a'='ita'
'ita' placed around 'o' is 'IOTA'.
(Other definitions for iota that I've seen before include "Extremely small amount" , "Ninth letter of Greek alphabet, small amount" , "Tiny bit" , "Gr. letter; small amount" , "Tiniest scrap" .)