In Caerphilly, it's not really mountainous (5)

I believe the answer is:

hilly

I'm a little stuck... Click here to teach me more about this clue!

'really mountainous' is the definition.
'hilly' can be an answer for 'mountainous' (synonyms). I'm unsure of the 'really' bit.

'in caerphilly it's not' is the wordplay.
'in' says the answer is hidden in the clue.
'HILLY' can be found hidden inside 'caerphilly not'.

Can you help me to learn more?

(Other definitions for hilly that I've seen before include "Unlike flat land" , "Descriptive of undulating upland" , "(Of a landscape) not flat" , "Like Chiltern or Cotswolds areas" , "Mountainous" .)

I've seen this clue in the Evening Standard.

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