Write out a couple of letters for a nut (5)
I believe the answer is:
pecan
'for a nut' is the definition.
(type of nut)
'write out a couple of letters' is the wordplay.
'a couple of letters' can mean two substitutions for 'letter'.
'write' becomes 'pen' (synonyms).
'out' means one lot of letters goes inside another.
'letter' becomes 'c' ().
'letter' becomes 'a'.
'c'+'a'='ca'
'pen' enclosing 'ca' is 'PECAN'.
(Other definitions for pecan that I've seen before include "Walnut-like nut" , "US nut tree" , "Species of hickory - edible nut" , "Nutty type" , "nut-tree" .)