Lead poisoning primarily leads to backache (8)
I believe the answer is:
plumbago
'lead' is the definition.
(I've seen this before)
'poisoning primarily leads to backache' is the wordplay.
'primarily' says to take the initial letters.
'leads to' says to put letters next to each other.
'backache' becomes 'lumbago' (lumbago is a kind of backache).
The first letter of 'poisoning' is 'p'.
'p'+'lumbago'='PLUMBAGO'
(Other definitions for plumbago that I've seen before include "Shrub; graphite" , "lead" , "Plant type that includes sea pinks" , "Word for graphite" , "Blue-flowered plant" .)