Unhappy Reginald, worried, losing head (9)
I believe the answer is:
regretful
'unhappy' is the definition.
(similar in meaning)
'reginald worried losing head' is the wordplay.
'reginald' becomes 'Reg' (short for Reginald).
'worried' becomes 'fretful' (fretful means worried or anxious).
'losing head' means to remove the first letter.
'fretful' with its initial letter removed is 'retful'.
'reg'+'retful'='REGRETFUL'
(Other definitions for regretful that I've seen before include "Miss Otis was" , "but not Ms Piaf" , "Wishing something hadn't happened" , "penitent" , "Feeling remorse" .)