Look in after offer is undercut (10)
I believe the answer is:
tenderloin
'undercut' is the definition.
('tenderloin' can be a synonym of 'undercut')
'look in after offer' is the wordplay.
'look' becomes 'lo'.
'after' says to put letters next to each other.
'offer' becomes 'tender' (tender offer is a kind of offer**).
'lo'+'in'='loin'
'loin' after 'tender' is 'TENDERLOIN'.
'is' acts as a link.
(Other definitions for tenderloin that I've seen before include "high crime rate here" , "Prime cut of pork, etc" , "seedy area of New York" , "soft meat" , "best cut" .)