Perhaps one playful Ulsterman's no saint (7)
I believe the answer is:
numeral
'perhaps one' is the definition.
(I know that 1 is a numeral)
'playful ulsterman's no saint' is the wordplay.
'playful' indicates an anagram.
'no' indicates named letters should be taken away.
'saint' becomes 'st' (abbreviation).
'ulsterman' with 'st' removed is 'ulerman'.
'ulerman' with letters rearranged gives 'NUMERAL'.
(Other definitions for numeral that I've seen before include "one involved in maths?" , "Symbol representing one or two, perhaps?" , "Such as 5, 9, iii, etc" , "Figure denoting a number" , "Symbol for number" .)