Not one in a hundred, strangely, could be a Parliamentarian! (9)
I believe the answer is:
roundhead
'a parliamentarian' is the definition.
(I've seen this before)
'not one in a hundred strangely' is the wordplay.
I cannot really understand how this works, but
'not' could be 'un' (I've seen this in other clues) and 'un' is found in the answer.
'one' could be 'a' and 'a' is present in the answer.
This explanation may well be incorrect...
'could be' is the link.
Can you help me to learn more?
(Other definitions for roundhead that I've seen before include "Supporter of Parliament during the English Civil Wars" , "Old soldier" , "An enemy of Charles" , "Cavalier's foe" , "antimonarchist" .)