Puritan, perhaps, but less than upright (4)
I believe the answer is:
prig
'puritan' is the definition.
(both can mean a prudish person)
'less than upright' is the wordplay.
'less' indicates a hidden word (the clue less some of the letters in it).
'PRIG' is hidden in the letters of 'than upright'.
'perhaps but' is the link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for prig that I've seen before include "Annoyingly arrogant person" , "humbug" , "Smugly self-righteous person" , "Smug type" , "Self-righteous hypocrite" .)