Attach large sign (4)
I believe the answer is:
link
'attach' is the definition.
(I know that attach is a more specific form of link)
'large sign' is the wordplay.
'large' becomes 'L' (eg in clothes sizes).
'sign' becomes 'ink' (ink can mean to sign a contract).
'l'+'ink'='LINK'
(Other definitions for link that I've seen before include "Tie-up" , "Physically join" , "Join together" , "Bond" , "Item of mail" .)