In certain cases, an old ruler (4)
I believe the answer is:
inca
'old ruler' is the definition.
(I've seen this before)
'in certain cases an' is the wordplay.
'in' indicates a hidden word.
'INCA' can be found hidden inside 'certain cases an'.
(Other definitions for inca that I've seen before include "Ancient Peruvian Indian" , "Member of ancient Peruvian tribe" , "Old South American native" , "Ruler of an old empire centred on Cuzco" , "Pre-conquest South American Indian" .)