They carry flags for model joists (8,7)
I believe the answer is:
standard bearers
'they carry flags' is the definition.
The definition suggests an adverb but the answer is not.
'model joists' is the wordplay.
'model' becomes 'standard' (model can mean a standard or template).
'joists' becomes 'bearers' (I can't explain this - if you can you should give a lot more credence to this answer).
'standard'+'bearers'='STANDARD-BEARERS'
'for' acts as a link.
Can you help me to learn more?
(Other definitions for standard bearers that I've seen before include "outstanding leaders" , "party leaders" .)