Principal pain? (8)

I believe the answer is:

headache

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'principal pain?' is the definition.
'headache' can be an answer for 'pain?' (headache is a kind of pain). I'm unsure of the 'principal' bit.

'principal pain?' is the wordplay.
'principal' becomes 'head' (synonyms).
'pain?' becomes 'ache' (ache is a kind of pain).
'head'+'ache'='HEADACHE'

Can you help me to learn more?

(Other definitions for headache that I've seen before include "Serious problem?" , "trial" , "Migraine" , "Informally a worrying problem - take an asprin" , "Worry, trouble (colloq.)" .)

I've seen this clue in The Telegraph.

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