Lean, father eats dinner finally (4)
I believe the answer is:
'lean' is the definition.
(I've seen this before)
'father eats dinner finally' is the wordplay.
'father' becomes 'pop' (pop is a kind of father).
'eats' means one lot of letters goes inside another.
'finally' says to take the final letters.
The last letter of 'dinner' is 'r'.
'pop' going around 'r' is 'PROP'.
(Other definitions for prop that I've seen before include "player involved in scrum" , "A rigid support - 37 Across is one" , "Front row forward" , "Temporary support" , "Seize and hold firmly" .)