Blow up an extension (11)
I believe the answer is:
enlargement
'an extension' is the definition.
(thesaurus)
'blow up' is the wordplay.
I cannot really see how this works, but
'blow up' could be 'enlarge' (synonyms) and 'enlarge' is found in the answer.
The remaining letters 'ment' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for enlargement that I've seen before include "Blown-up picture" , "Blow-up photograph" , "Blown-up photograph" , "Photographic blow-up" , "Blown-up photo" .)