He’s crotchety and has a bad cold in the head (5)
I believe the answer is:
crank
'he's crotchety and has a bad' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'cold in the head' is the wordplay.
'cold' becomes 'c' (eg on taps).
'in the head' becomes 'rank' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'c'+'rank'='CRANK'
Can you help me to learn more?
(Other definitions for crank that I've seen before include "Turn (engine handle); eccentric" , "Person of eccentric habits" , "Turn a handle - weirdo" , "Person with strange ideas, or machine part" , "Wind (up) - oddball" .)