A reversal in function makes us gloomy (9)
I believe the answer is:
saturnine
'gloomy' is the definition.
(I've seen this before)
'a reversal in function' is the wordplay.
'reversal' becomes 'turn' (turning around).
'in' indicates putting letters inside.
'function' becomes 'sine' (mathematical function).
'a'+'turn'='aturn'
'aturn' put inside 'sine' is 'SATURNINE'.
'makes us' is the link.
(Other definitions for saturnine that I've seen before include "Morose looking" , "Gloomy (temperament)" , "moody" , "may be brooding" , "shining? The opposite" .)