Expansion of the novel 'Greenmantle' (11)
I believe the answer is:
enlargement
'expansion' is the definition.
('enlargement' can be a synonym of 'expansion')
'the novel greenmantle' is the wordplay.
'the novel' indicates an anagram.
'greenmantle' anagrammed gives 'ENLARGEMENT'.
'of' is the link.
(Other definitions for enlargement that I've seen before include "Blow-up photo" , "Blow-up photograph" , "Bigger version of photographic print" , "Blown up print" , "increase" .)