More troubled by exam, becoming most withdrawn (8)
I believe the answer is:
remotest
'most withdrawn' is the definition.
'more troubled by exam' is the wordplay.
'troubled' indicates an anagram.
'by' means one lot of letters go next to another.
'exam' becomes 'test' (synonyms).
'more' with letters rearranged gives 'remo'.
'remo'+'test'='REMOTEST'
'becoming' is the link.
(Other definitions for remotest that I've seen before include "Terse mot (anag)" , "Furthest away, most distant" .)