Regularly use stirrups as a supporting framework (5)
I believe the answer is:
truss
'supporting framework' is the definition.
(I've seen this before)
'regularly use stirrups as a' is the wordplay.
'regularly use' means one should take alternating letters (I've seen 'regularly used' mean this (letters selected on a regular basis)).
'stirrups'+'as'+'a'='stirrupsasa'
The alternate letters of 'stirrupsasa' are 'TRUSS'.
(Other definitions for truss that I've seen before include "Medical support" , "Type of support" , "Tie up - bind" , "Bind like a chicken" , "Frame supporting roof" .)