Dimension of certain chaps in the flesh (11)
I believe the answer is:
measurement
'dimension' is the definition.
(I've seen this before)
'certain chaps in the flesh' is the wordplay.
'certain' becomes 'sure' (synonyms).
'chaps' becomes 'men' (chap can mean a man).
'in the' means one lot of letters goes inside another.
'flesh' becomes 'meat' (flesh is meat).
'sure'+'men'='suremen'
'suremen' going into 'meat' is 'MEASUREMENT'.
'of' acts as a link.
(Other definitions for measurement that I've seen before include "Mensuration" , "What a ruler may give" , "Size" , "Dimension" .)