Former substitute could be demanding (8)
I believe the answer is:
exacting
'be demanding' is the definition.
('exacting' can be similar in meaning to 'demanding')
'former substitute' is the wordplay.
'former' becomes 'ex'.
'substitute' becomes 'acting' (e.g. acting president**).
'ex'+'acting'='EXACTING'
'could' is the link.
(Other definitions for exacting that I've seen before include "Demanding great skill" , "Requiring great effort, arduous" , "Tough" , "Requiring great effort - sounds precise" , "Stringent" .)