Trained to nurse sore head and pain in the neck (4)
I believe the answer is:
pest
'pain in the neck' is the definition.
(I've seen this in another clue)
'trained to nurse sore head' is the wordplay.
'trained' becomes 'pet' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'to nurse' is an insertion indicator.
'head' indicates taking the first letters.
The first letter of 'sore' is 's'.
'pet' enclosing 's' is 'PEST'.
'and' acts as a link.
Can you help me to learn more?
(Other definitions for pest that I've seen before include "Destructive insect or other animal" , "Plague" , "Annoying creature" , "Troublesome person" , "Nuisance; half a European capital" .)