Float art gallery supporting garment-maker (8)
I believe the answer is:
levitate
'float' is the definition.
(I've seen this before)
'art gallery supporting garment-maker' is the wordplay.
'art gallery' becomes 'tate' (Tate Modern, Tate Britain etc).
'supporting' is a charade indicator (letters next to each other) (in a down clue, some letters go below others).
'garment-maker' becomes 'levi' (I can't explain this - if you can you should believe this answer much more).
'tate' after 'levi' is 'LEVITATE'.
Can you help me to learn more?
(Other definitions for levitate that I've seen before include "Rise in defiance of gravity for vital tee" , "Rise into the air by magic?" , "Rise and float in defiance of gravity" , "Float magically in air" , "Rise in the air, unsupported" .)