Saint engaged in bowel movement (4)
I believe the answer is:
elmo
'saint' is the definition.
(I have seen 'Fiery saint' mean 'elmo' so perhaps 'saint' could also mean 'elmo')
'engaged in bowel movement' is the wordplay.
'engaged in' indicates a hidden word.
'ELMO' can be found hidden inside 'bowel movement'.
(Other definitions for elmo that I've seen before include "Sesame Street character - patron saint of sailors (with his own fire)" , "Saint associated with fire" , "St ...'s fire, weather phenomenon of coloured glow" , "Fiery saint" , "Puppet character" .)