Pastime by shore involved toy (10)
I believe the answer is:
hobbyhorse
'pastime' is the definition.
(thesaurus)
'by shore involved toy' is the wordplay.
'by' is a charade indicator (letters next to each other).
'involved' indicates an anagram (involved can mean confusing or complex).
'toy' becomes 'hobby' (hobby is a kind of toy).
'shore' anagrammed gives 'horse'.
'horse' after 'hobby' is 'HOBBYHORSE'.
(Other definitions for hobbyhorse that I've seen before include "Toy" , "plaything" .)