It’s twaddle, so go back in and rest (5)
I believe the answer is:
roost
'rest' is the definition.
(synonyms)
'it's twaddle so go back in' is the wordplay.
'it's twaddle' becomes 'rot' (I've seen this before).
'go back' shows that the letters should be reversed in order.
'in' means one lot of letters goes inside another.
'so' back-to-front is 'os'.
'rot' placed around 'os' is 'ROOST'.
'and' acts as a link.
(Other definitions for roost that I've seen before include "Resting perch for birds" , "Temporary resting-place" , "Bird's night spot" , "(Bird) settle for night" , "Where bats congregate" .)