Was standing in exercise the wrong way, caught in act (9)
I believe the answer is:
deputised
'was standing in' is the definition.
(deputise can mean to stand in for)
'exercise the wrong way caught in act' is the wordplay.
'exercise' becomes 'situp' (type of exercise).
'the wrong way' shows that the letters should be reversed in order.
'caught in' means one lot of letters goes inside another.
'act' becomes 'deed' (deed is a kind of act).
'situp' reversed gives 'putis'.
'putis' placed inside 'deed' is 'DEPUTISED'.
(Another definition for deputised that I've seen is " Stood in (for)".)