Idiot claiming trophy is limiting opening in computer science (12)
I believe the answer is:
astrophysics
'science' is the definition.
(astrophysics is a kind of science)
'idiot claiming trophy is limiting opening in computer' is the wordplay.
'idiot' becomes 'ass' (both can mean a stupid person).
'claiming' means one lot of letters goes inside another.
'limiting' is an insertion indicator.
'opening in' suggests taking the first letters.
The initial letter of 'computer' is 'c'.
'ass' enclosing 'trophy' is 'astrophys'.
'is' enclosing 'c' is 'ics'.
'astrophys'+'ics'='ASTROPHYSICS'
(Other definitions for astrophysics that I've seen before include "Science of the properties of celestial bodies" , "Science of stars and galaxies" , "Branch of science" .)