NHS Apr pay reviews -- a pain in the backside (5,4)

I believe the answer is:

nappy rash

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

'a pain in the backside' is the definition.
I know nothing about this answer so I cannot judge whether this works.

'nhs apr pay reviews' is the wordplay.
'reviews' indicates an anagram (I've seen 'review' mean this).
'nhs'+'apr'+'pay'='nhsaprpay'
'nhsaprpay' is an anagram of 'NAPPY RASH'.

Can you help me to learn more?

(Other definitions for nappy rash that I've seen before include "wet spots" , "Baby's discomfort" .)

I've seen this clue in The Telegraph.

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