Unusual thing for artist regularly absorbed by fish (6)
I believe the answer is:
rarity
'unusual thing' is the definition.
(I've seen this before)
'artist regularly absorbed by fish' is the wordplay.
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
'absorbed by' means one lot of letters goes inside another.
'fish' becomes 'ray' (ray is a kind of fish).
The alternating letters of 'artist' are 'rit'.
'rit' inserted inside 'ray' is 'RARITY'.
'for' acts as a link.
(Other definitions for rarity that I've seen before include "Find" , "something out of the ordinary" , "Collector's item" , "Unusual thing" , "Uncommon thing" .)