Support I’ll back, beset by endless stress (7)
I believe the answer is:
trellis
'support' is the definition.
'i'll back beset by endless stress' is the wordplay.
'back' says the letters should be written backwards.
'beset by' is an insertion indicator ('beset' could historically mean 'cover').
'endless' says to take the centre.
The centre of 'stress' is 'tres'.
'ill' back-to-front is 'lli'.
'lli' going within 'tres' is 'TRELLIS'.
(Other definitions for trellis that I've seen before include "Light framework for climbing plants" , "Garden climbing frame" , "trainer" , "supporting frame" , "Arch of latticework" .)