Photo of generous chaps in English National Trust (11)
I believe the answer is:
enlargement
'photo' is the definition.
(I know that enlargement is a type of photo)
'generous chaps in english national trust' is the wordplay.
'generous' becomes 'large' (similar in meaning).
'chaps' becomes 'men' (chap can mean a man).
'in' indicates putting letters inside.
'english' becomes 'E' (abbreviation as in OED).
'national trust' becomes 'nt'.
'large'+'men'='largemen'
'e'+'nt'='ent'
'largemen' put within 'ent' is 'ENLARGEMENT'.
'of' acts as a link.
(Other definitions for enlargement that I've seen before include "Blown-up photo" , "Blow-up photograph" , "increase" , "Blow-up photo" , "Blown-up photograph" .)