Endlessly longing for a headache cure (7)
I believe the answer is:
aspirin
'headache cure' is the definition.
(I've seen this before)
'endlessly longing for a' is the wordplay.
'endlessly' says to take the centre (the ends of the words are removed).
'longing' becomes 'aspiring' (aspiring to something means longing for it).
'for' says to put letters next to each other (I've seen this in other clues).
The central letters of 'aspiring' are 'spirin'.
'spirin' put after 'a' is 'ASPIRIN'.
(Other definitions for aspirin that I've seen before include "Sip rain (anag.)" , "Common analgesic drug" , "Anti-inflammatory analgesic" , "Pain-relieving drug made in Paris" , "Headache killer?" .)