Support letters being rewritten (7)
I believe the answer is:
trestle
'support' is the definition.
(I've seen this before)
'letters being rewritten' is the wordplay.
'being rewritten' indicates an anagram.
'letters' is an anagram of 'TRESTLE'.
(Other definitions for trestle that I've seen before include "support of growth" , "horse" , "Supporting framework for, eg, table top" , "Frame supporting table" , "Frame to hold table" .)