Smooth-tongued learner ignored head's complaint (8)
I believe the answer is:
sickness
'complaint' is the definition.
(sickness is a kind of health problem)
'smooth-tongued learner ignored head's' is the wordplay.
'smooth-tongued' becomes 'slick'.
'learner' becomes 'l' (as in a learner driver with L-plates on their car).
'ignored' is a deletion indicator.
'head' becomes 'ness' (ness is a headland).
'slick' with 'l' removed is 'sick'.
'sick'+'ness'='SICKNESS'
(Other definitions for sickness that I've seen before include "Infirmity" , "Ill health" , "State of ill-health, malady" , "State of being ill" , "Illness, disease" .)