Supporter lacking in bravery: extremely (4)
I believe the answer is:
very
'extremely' is the definition.
(thesaurus)
'supporter lacking in bravery' is the wordplay.
'supporter' becomes 'bra' (I've seen this before).
'lacking in' indicates named letters should be taken away.
'bravery' with 'bra' taken out is 'VERY'.
(Other definitions for very that I've seen before include "Frightfully" , "Kind of light" , "Ever so" , "Truly" , "Absolutely" .)