Tweet letter: pleased Tom might do this in retirement (7)
I believe the answer is:
chirrup
'tweet' is the definition.
(I know that tweet can be written as chirrup)
'letter pleased tom might do this in retirement' is the wordplay.
'letter' becomes 'chi' (chi is a kind of letter).
'pleased tom might do this' becomes 'purr' (tom can mean a male cat).
'in retirement' shows that the letters should be reversed in order (retire can mean to go back or backwards).
'purr' written backwards gives 'rrup'.
'chi'+'rrup'='CHIRRUP'
(Other definitions for chirrup that I've seen before include "Sound of small bird" , "Make high-pitched sounds" , "Bird's noise" , "Noise of a small bird" , "Twitter" .)