Fantasises a lot after diamonds (6)
I believe the answer is:
dreams
'fantasises' is the definition.
'a lot after diamonds' is the wordplay.
'a lot' becomes 'reams'.
'after' means one lot of letters go next to another.
'diamonds' becomes 'd' (abbreviation in bridge say).
'reams' put after 'd' is 'DREAMS'.
(Other definitions for dreams that I've seen before include "Imaginings" , "Fantasy ambitions" , "'We are such stuff / As dreams are made on' (The Tempest)" , "Fantasies; fantasises" , "Fancies" .)