University blocking mass influence (5)
I believe the answer is:
clout
'influence' is the definition.
(I've seen this in another clue)
'university blocking mass' is the wordplay.
'university' becomes 'U' (abbreviation for university).
'blocking' means one lot of letters goes inside another (to block can mean to plug a gap in something).
'mass' becomes 'clot' (I have seen 'Mass of material' mean 'clot' so perhaps 'mass' could also mean 'clot').
'u' put inside 'clot' is 'CLOUT'.
(Other definitions for clout that I've seen before include "Old-fashioned garment" , "Box" , "Sock" , "A blow; powerful influence" , "Cuff" .)