What mustn't be done with two legs elevated (2-2)
I believe the answer is:
no-no
'what mustn't be done' is the definition.
(a no-no is something ill-advised)
'with two legs elevated' is the wordplay.
'two legs' can mean two replacements for 'leg'.
'with' says to put letters next to each other.
'leg' becomes 'on' (cricket term, fielding positions).
'leg' becomes 'on' (cricket term, fielding positions).
'elevated' shows that the letters should be reversed in order (down clues only).
'on' put next to 'on' is 'onon'.
'onon' written backwards gives 'NO-NO'.
(Other definitions for no-no that I've seen before include "Something that would be totally unacceptable" , "turn-off for some?" , "Impossibility" , "Something that must not be done (informal)" , "Non-starter" .)