Unnecessary to sound off about day with nothing on in retreat (9)
I believe the answer is:
redundant
'unnecessary' is the definition.
(similar in meaning)
'sound off about day with nothing on in retreat' is the wordplay.
'sound off' becomes 'rant' (to sound off is to rant or complain).
'about' means one lot of letters goes inside another.
'day' becomes 'd' (abbreviation for day).
'with nothing on' becomes 'nude' (without clothes on).
'in retreat' is a reversal indicator.
'd'+'nude'='dnude'
'dnude' written backwards gives 'edund'.
'rant' going around 'edund' is 'REDUNDANT'.
'to' is the link.
(Other definitions for redundant that I've seen before include "No longer necessary" , "No longer required, unnecessary" , "jobless!" , "no longer wanted" , "No longer useful" .)