Waste time on a letter to a friend (5)
I believe the answer is:
dally
'waste time' is the definition.
(I've seen this in another clue)
'on a letter to a friend' is the wordplay.
'on' means one lot of letters go next to another.
'a letter' becomes 'd' ().
'to a friend' becomes 'ally' (an ally is a friend or supporter).
'd' put next to 'ally' is 'DALLY'.
(Other definitions for dally that I've seen before include "Move slowly" , "Procrastinate" , "Trifle" , "Linger" , "Waste time" .)