Dirtbag occasionally pens love letter (4)
I believe the answer is:
iota
'letter' is the definition.
(Greek letter)
'dirtbag occasionally pens love' is the wordplay.
'occasionally' indicates alternate letters (letters taken at regular intervals or occasions).
'pens' means one lot of letters goes inside another (inserted letters are held or penned in).
'love' becomes 'o' (love means zero in tennis).
The alternate letters of 'dirtbag' are 'ita'.
'ita' placed around 'o' is 'IOTA'.
(Other definitions for iota that I've seen before include "The Greek i (4)" , "Just a jot, from Greek alphabet" , "Trace" , "Greek letter - small quantity" , "Foreign character" .)