The reduced close isn't as broad (7)
I believe the answer is:
thinner
'broad' is the definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.
'the reduced close isn't' is the wordplay.
'reduced' means to remove the last letter.
'close isn't' becomes 'inner' (I can't explain this - if you can you should give a lot more credence to this answer).
'the' with its final letter taken off is 'th'.
'th'+'inner'='THINNER'
'as' acts as a link.
Can you help me to learn more?
(Other definitions for thinner that I've seen before include "Diluting solvent" , "what one adds to paint" , "Leaner" , "Slenderer" , "Slimmer" .)