TV doctor's friend giving up acting entirely (6)
I believe the answer is:
wholly
'entirely' is the definition.
(I know that entirely can be written as wholly)
'tv doctor's friend giving up acting' is the wordplay.
'tv doctor' becomes 'who' (Doctor Who).
'friend' becomes 'ally' (an ally is a friend or supporter).
'giving up' suggests deleting specific letters.
'acting' becomes 'a' (abbreviation, eg in job titles).
'ally' with 'a' taken out is 'lly'.
'who'+'lly'='WHOLLY'
(Other definitions for wholly that I've seen before include "Without exception" , "Absolutely!" , "in every area" , "in full?" , "Entirely, fully" .)