Almost becoming a candidate when you act as substitute for (5,2)
I believe the answer is:
stand in
'substitute for' is the definition.
'stand in' can be an answer for 'substitute' (synonyms). I'm unsure of the 'for' bit.
'almost becoming a candidate when you act' is the wordplay.
I cannot really understand how this works, but
'a' could be 'an' and 'an' is located in the answer.
'act' could be 'din' (din is a kind of act) and 'din' is located in the answer.
The remaining letters 'st' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
'as' is the link.
Can you help me to learn more?
(Other definitions for stand in that I've seen before include "Double" , "Substitute or deputy" , "Vocal pieces" , "Sub" , "Substitute's" .)