Gloomy sort of cleaner, about to retire (5)
I believe the answer is:
moper
'gloomy sort' is the definition.
The definition suggests a singular noun which matches the answer.
'cleaner about to retire' is the wordplay.
'cleaner' becomes 'mop' (something which 'cleans').
'about' becomes 're' (regarding).
'to retire' shows that the letters should be reversed in order (retire can mean to go back).
're' in reverse letter order is 'er'.
'mop'+'er'='MOPER'
'of' is the link.
Can you help me to learn more?
(Other definitions for moper that I've seen before include "One who's depressed" , "Weary Willie" .)