8 down
Pine
nut’s
beginning
to
be
problem
(8)
Pine nut’s beginning to be problem (8)
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The definition is at the end of the clue.
How the clue works
The definition is problem, since a headache is a source of trouble. The answer is built from HEAD (a slang term for 'nut') followed by ACHE (to 'pine' or long for), giving HEAD+ACHE.