Opening Henry found in Riviera resort (5)
I believe the answer is:
niche
'opening' is the definition.
'henry found in riviera resort' is the wordplay.
'henry' becomes 'H' (symbol for the unit of electrical inductance).
'found in' means one lot of letters goes inside another.
'riviera resort' becomes 'nice' (I've seen this in another clue).
'h' placed within 'nice' is 'NICHE'.
(Other definitions for niche that I've seen before include "Nook or recess" , "Place that's cosy" , "Suitable location" , "Comfortable position" , "Calling" .)