Member or Leader of Parliament, say (3)
I believe the answer is:
peg
'member' is the definition.
(peg is a kind of member)
'leader of parliament say' is the wordplay.
'leader of' indicates taking the first letters.
'say' becomes 'eg'.
The first letter of 'parliament' is 'p'.
'p'+'eg'='PEG'
'or' acts as a link.
(Other definitions for peg that I've seen before include "Limit" , "This leg is artificial" , "Small cylindrical pin" , "Fix (a price)" , "Kind of pin for hanging clothes" .)