Support it from beneath, using tack (8)
I believe the answer is:
underpin
'support' is the definition.
(I know that underpin is a more specific form of support)
'beneath using tack' is the wordplay.
'beneath' becomes 'under' (under means beneath or underneath).
'using' means one lot of letters go next to another.
'tack' becomes 'pin' (a tack is a nail or pin).
'under'+'pin'='UNDERPIN'
'it from' is the link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for underpin that I've seen before include "Form the basis for" , "Prop up with nun pride" , "Support or strengthen from beneath" , "buttress" , "corroborate" .)