Fleet, American -- not all there captured by artist (6)
I believe the answer is:
armada
'fleet' is the definition.
(I know that armada is a type of fleet)
'american not all there captured by artist' is the wordplay.
'american' becomes 'a' (common abbreviation - e.g. in organisation names).
'not all there' becomes 'mad' (I've seen this in another clue).
'captured by' is an insertion indicator.
'artist' becomes 'RA' (Royal Academician, member of Royal Academy of Arts).
'mad' going within 'ra' is 'rmada'.
'a'+'rmada'='ARMADA'
(Other definitions for armada that I've seen before include "Spanish fleet sent in 1588 to escort an invasion of England" , "Large war fleet" , "A great fleet, ships or planes" , "Large number of ships (Spanish?)" , "Large Spanish fleet" .)