A lonely one lacking a supporter (4)
I believe the answer is:
ally
'a supporter' is the definition.
(an ally is a friend or supporter)
'a lonely one lacking' is the wordplay.
'lacking' is a deletion indicator.
'lonely' with 'one' removed is 'lly'.
'a'+'lly'='ALLY'
(Other definitions for ally that I've seen before include "Colleague, confederate" , "often helpful when cluing adverbs" , "Co-operating person or nation" , "Friend, partner" , "couple?" .)