Tablet has no power with head ailment (7)
I believe the answer is:
illness
'ailment' is the definition.
(thesaurus)
'tablet has no power with head' is the wordplay.
'tablet' becomes 'pill' ('pill' can be a synonym of 'tablet').
'has no' suggests deleting specific letters.
'power' becomes 'p'.
'with' says to put letters next to each other.
'head' becomes 'ness' (ness is a headland**).
'pill' with 'p' taken out is 'ill'.
'ill'+'ness'='ILLNESS'
(Other definitions for illness that I've seen before include "Malady, sickness" , "Sick condition" , "Indisposition, sickness" , "Complaint" , "suffering this" .)