Four unknown to creeper (3)
I believe the answer is:
ivy
'creeper' is the definition.
(I've seen this in another clue)
'four unknown' is the wordplay.
'four' becomes 'IV' (Roman numerals).
'unknown' becomes 'y' (algebra).
'iv'+'y'='IVY'
'to' is the link.
(Other definitions for ivy that I've seen before include "Hedera helix" , "Bush" , "Creeper plant" , "The ... League comprises USA's top universities" , "Poisonous plant" .)