Pleasant, having to return poster (6)
I believe the answer is:
notice
'poster' is the definition.
(I know that poster can be written as notice)
'pleasant having to return' is the wordplay.
'pleasant' becomes 'nice' (similar in meaning).
'having' means one lot of letters goes inside another.
'return' says the letters should be written in reverse.
'to' back-to-front is 'ot'.
'nice' going around 'ot' is 'NOTICE'.
(Other definitions for notice that I've seen before include "Detect" , "Observe or regard" , "Advance indication of intention to resign" , "Mark" , "Notification" .)