Perform inadequately as less fancied competitor? Not good (7)
I believe the answer is:
underdo
'perform inadequately' is the definition.
'less fancied competitor? not good' is the wordplay.
'less fancied competitor?' becomes 'underdog' (I've seen this before).
'not' suggests deleting specific letters.
'good' becomes 'g' (abbreviation).
'underdog' with 'g' taken away is 'UNDERDO'.
'as' is the link.
(Other definitions for underdo that I've seen before include "Be responsible for rare product" , "Cook too briefly" , "Cook in a bad way" , "Rarely cook" , "Cook for short time" .)