Support pawns capturing rook, oddly (4)
I believe the answer is:
prop
'support' is the definition.
(prop up can mean to support)
'pawns capturing rook oddly' is the wordplay.
'pawns' becomes 'pp' (chess abbreviation for pawn written twice).
'capturing' means one lot of letters goes inside another.
'oddly' indicates alternate letters (letters in the odd-numbered positions are used).
The alternating letters of 'rook' are 'ro'.
'pp' enclosing 'ro' is 'PROP'.
(Other definitions for prop that I've seen before include "Back up" , "Temporary support" , "Support - item on stage" , "Set piece" , "Driving mechanism" .)