Cause of discomfort, one involving head, principally (4)
I believe the answer is:
ache
'cause of discomfort' is the definition.
'ache' can be an answer for 'discomfort' (I've seen this in another clue). I am not sure about the 'cause of' bit.
'one involving head principally' is the wordplay.
'one' becomes 'ace' (ace has a value of one in some card games).
'involving' indicates putting letters inside.
'principally' suggests taking the first letters.
The initial letter of 'head' is 'h'.
'ace' enclosing 'h' is 'ACHE'.
Can you help me to learn more?
(Other definitions for ache that I've seen before include "Dull prolonged pain" , "yearning" , "Persistent dull pain" , "Throb - hurt" , "Similar" .)