Knight, behold, has popular beef (7)
I believe the answer is:
sirloin
'beef' is the definition.
(I know that sirloin is a type of beef)
'knight behold has popular' is the wordplay.
'knight' becomes 'sir' (a knight has the title Sir).
'behold' becomes 'lo' (archaic exclamation meaning 'look!').
'has' means one lot of letters go next to another.
'popular' becomes 'in' ('in' can mean trendy).
'sir'+'lo'+'in'='SIRLOIN'
(Other definitions for sirloin that I've seen before include "(Knighted?) cut of meat" , "Good cut of beef" , "Type of steak" , "Choice cut of beef" , "Upper part of loin of beef" .)