Queen's piercing version of 'Imagine' -- headache! (8)
I believe the answer is:
migraine
'headache' is the definition.
(I know that migraine is a type of headache)
'queen's piercing version of imagine' is the wordplay.
'queen' becomes 'r' (abbreviation for regina, e.g. in ER ).
'piercing' means one lot of letters goes inside another (some letters pierce a hole in a word).
'version of' indicates an anagram (another version of the same letters).
'imagine' anagrammed gives 'migaine'.
'r' inserted within 'migaine' is 'MIGRAINE'.
(Other definitions for migraine that I've seen before include "attack" , "terrible pain" , "Crippling headache" , "A really bad headache" , "Long nerve fibre" .)