Fits in with trick shapes (8)
I believe the answer is:
conforms
'fits' is the definition.
(conforming is a kind of fitting)
'with trick shapes' is the wordplay.
'with' says to put letters next to each other.
'trick' becomes 'con' (a con is a fraud or trick).
'shapes' becomes 'forms' ('form' can be a synonym of 'shape').
'con'+'forms' is 'CONFORMS'.
'in' acts as a link.
(Other definitions for conforms that I've seen before include "Keeps to rules or customs (8)." , "Toes the party line" , "Keeps to rules or general custom" .)