Huge precious stone mounted on middle of tiara (4)
I believe the answer is:
mega
'huge' is the definition.
(I've seen this before)
'precious stone mounted on middle of tiara' is the wordplay.
'precious stone' becomes 'gem' (gem is a kind of precious stone).
'mounted' shows that the letters should be reversed in order.
'on' says to put letters next to each other.
'middle of' means to look at the middle letters.
The centre of 'tiara' is 'a'.
'gem' back-to-front is 'meg'.
'meg'+'a'='MEGA'
(Other definitions for mega that I've seen before include "'Excellent, colloquially (4)'" , "Big" , "Enormous; very good" , "Huge (slang)" , "Terrific" .)