Fail to secure enough support (8)
I believe the answer is:
underpin
'enough support' is the definition.
'underpin' can be an answer for 'support' (underpinning is a kind of supporting). I am not certain of the 'enough' bit.
'fail to secure' is the wordplay.
'fail' becomes 'under' (I can't justify this - if you can you should believe this answer much more).
'to secure' becomes 'pin' (pinning is a kind of securing).
'under'+'pin'='UNDERPIN'
Can you help me to learn more?
(Other definitions for underpin that I've seen before include "buttress" , "Support or strengthen from beneath" , "Form the basis for" , "corroborate" , "Prop up with nun pride" .)