This gives the size of certain fellows in the flesh (11)
I believe the answer is:
measurement
'this gives the size' is the definition.
The definition suggests a singular noun which matches the answer.
'certain fellows in the flesh' is the wordplay.
'certain' becomes 'sure' (synonyms).
'fellows' becomes 'men' (fellow can mean a man).
'in the' indicates putting letters inside.
'flesh' becomes 'meat' (flesh is meat).
'sure'+'men'='suremen'
'suremen' placed inside 'meat' is 'MEASUREMENT'.
'of' acts as a link.
Can you help me to learn more?
(Other definitions for measurement that I've seen before include "What a ruler may give" , "Dimension" , "Mensuration" , "Size" .)