Support from beneath, subject to identification (8)
I believe the answer is:
underpin
'support from beneath' is the definition.
(I've seen this before)
'subject to identification' is the wordplay.
'subject to' becomes 'under' (being subject to a law is being under it).
'identification' becomes 'pin' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'under'+'pin'='UNDERPIN'
Can you help me to learn more?
(Other definitions for underpin that I've seen before include "Support or strengthen from beneath" , "Prop up with nun pride" , "Form the basis for" , "corroborate" , "buttress" .)