Fantastic, seeing a jewel mounted (4)
I believe the answer is:
mega
'fantastic' is the definition.
(I've seen this before)
'a jewel mounted' is the wordplay.
'jewel' becomes 'gem' (synonyms).
'mounted' says the letters should be written in reverse.
'a'+'gem'='agem'
'agem' in reverse letter order is 'MEGA'.
'seeing' is the link.
(Other definitions for mega that I've seen before include "Game (anag) - excellent" , "Fantastic!" , "Terrific" , "Very large (prefix)" , "Great - denoting a million" .)