Where one who’s in stands to gain (8)
I believe the answer is:
increase
'gain' is the definition.
(I know that gain can be written as increase)
I can't explain the remainder of the clue.
(Other definitions for increase that I've seen before include "Get bigger" , "Make larger in a sincere way" , "Grow, expand" , "What's more" , "Grow bigger" .)