Mass attended by Irish beside unknown river (6)
I believe the answer is:
'river' is the definition.
'mass attended by irish beside unknown' is the wordplay.
'mass' becomes 'm' (symbol used in Physics e.g. f=ma).
'attended by' says to put letters next to each other.
'irish' becomes 'erse' (I've seen this before).
'beside' means one lot of letters go next to another (I've seen this in other clues).
'unknown' becomes 'y' (algebra).
(Other definitions for mersey that I've seen before include "Runcorn's river" , "River flowing into Liverpool Bay" , "Scouse river" , "River with ferry and tunnel" , "River linking Greater Manchester and Liverpool" .)