The cereal I have picked up is a pain (8)
I believe the answer is:
migraine
'pain' is the definition.
(I know that migraine is a type of cephalalgia)
'the cereal i have picked up' is the wordplay.
I cannot quite see how this works, but
'cereal' could be 'grain' (synonyms) and 'grain' is present in the answer.
'i' is within the answer.
The remaining letters 'me' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
'is a' is the link.
(Other definitions for migraine that I've seen before include "From mare gin I get a severe headache" , "Severe recurring headache" , "misery in capital area" , "Throbbing inside the head" , "Disabling headache" .)