Put up all art necessarily held to be fundamental (7)
I believe the answer is:
central
'be fundamental' is the definition.
('central' can be a synonym of 'fundamental')
'put up all art necessarily held' is the wordplay.
'put up' says the letters should be written backwards.
'held' says the answer is hidden in the clue.
'lartnec' is hidden within 'all art necessarily'.
'lartnec' backwards is 'CENTRAL'.
'to' acts as a link.
(Other definitions for central that I've seen before include "Middle; crucial" , "like this answer" , "Most important, principal" , "Every answer here has one such letter" , "Key" .)