Stop holding back, saint is most dull (9)
I believe the answer is:
dreariest
'most dull' is the definition.
('dreary' can be similar in meaning to 'dull')
'stop holding back saint' is the wordplay.
I cannot really understand how this works, but
'back' could be 'rear' (synonyms) and 'rear' is found within the answer.
'stop' could be 'die' (I've seen this in other clues) and 'die' is present in the leftover letters.
'saint' could be 'st' and 'st' is located in the answer.
This accounts for all the letters.
This may be the basis of clue (or it may be nonsense).
'is' acts as a link.
(Other definitions for dreariest that I've seen before include "Most tiresome" , "Not the most exciting" .)