Surprisingly, a poem's seen to offer passcode (4,6)
I believe the answer is:
open sesame
'passcode' is the definition.
(Ali Baba and the Forty Thieves password)
'surprisingly a poem's seen' is the wordplay.
'surprisingly' indicates an anagram (a surprising ordering of the letters).
'a'+'poems'+'seen'='apoemsseen'
'apoemsseen' with letters rearranged gives 'OPEN SESAME'.
'to offer' is the link.
(Other definitions for open sesame that I've seen before include "spelling words?" , "Spell from Ali Baba and the Forty Thieves" , "a very successful means of achieving a result" , "free entry" , "Spell unlocking doors" .)