Mark avoids ruining Bankside pursuit (7)
I believe the answer is:
angling
'bankside pursuit' is the definition.
(fishing on the bank of a river)
'mark avoids ruining' is the wordplay.
'mark' becomes 'm'.
'avoids' is a deletion indicator.
'ruining' becomes 'mangling'.
'mangling' with 'm' removed is 'ANGLING'.
(Other definitions for angling that I've seen before include "Indirectly seeking" , "riverside activity?" , "Walton practised it" , "Pointing" , "seeking compliments, perhaps?" .)