Rising cheers surrounding Conservative legislation (3)
I believe the answer is:
act
'legislation' is the definition.
(I've seen this in other clues)
'rising cheers surrounding conservative' is the wordplay.
'rising' is a reversal indicator (going up in a down clue).
'cheers' becomes 'ta' (both can mean thank you).
'surrounding' is an insertion indicator.
'conservative' becomes 'C' (UK political abbreviation).
'ta' placed around 'c' is 'tca'.
'tca' written backwards gives 'ACT'.
(Other definitions for act that I've seen before include "Take part in a play" , "Thing done" , "Perform - part of a play" , "Do something, in a play?" , "Major division of play" .)