Rock a prince found in the Kent area (5)
I believe the answer is:
shale
'rock' is the definition.
(I know that shale is a type of sedimentary rock)
'a prince found in the kent area' is the wordplay.
'a prince' becomes 'hal' (Shakespearean nickname of Henry V when a prince).
'found in' means one lot of letters goes inside another.
'the kent area' becomes 'se' (Kent is in the South East of England).
'hal' placed into 'se' is 'SHALE'.
(Other definitions for shale that I've seen before include "Clayey rock" , "Oil-rich rock" , "Soft sedimentary rock" , "Stone that splits easily into fine pieces" , "Layered sedimentary rock" .)