Describing one who has been willing to set up an art gallery (7)
I believe the answer is:
testate
'describing one who has been willing' is the definition.
I can't tell whether this defines the answer.
'set up an art gallery' is the wordplay.
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
'an art gallery' becomes 'tate' (Tate Modern, Tate Britain etc).
'set' in reverse letter order is 'tes'.
'tes'+'tate'='TESTATE'
'to' acts as a link.
Can you help me to learn more?
(Other definitions for testate that I've seen before include "With a legally valid will" , "Having made a valid will" , "Having made a will before death" , "Dying with a will" , "Having made a legal will" .)