He composes a letter to suit (5)
I believe the answer is:
titus
'he' is the definition.
'composes a letter to suit' is the wordplay.
I cannot really understand how this works, but
'letter' could be 'u' and 'u' is present in the answer.
The remaining letters 'tits' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for titus that I've seen before include "Roman emperor with arch" , "Groan, say" , "saint" , "A Roman emperor; a Pauline addressee" , "identifying Philemon's neighbour" .)