Early high-flier, one occupying river bed (4,5)
I believe the answer is:
test pilot
'early high-flier' is the definition.
The definition suggests a singular noun which matches the answer.
'one occupying river bed' is the wordplay.
'one' becomes 'i' (Roman numeral).
'occupying' is an insertion indicator (inserted letters occupy or enter the word).
'river' becomes 'test' (the River Test).
'bed' becomes 'plot' (a bed or plot of land).
'test'+'plot'='testplot'
'i' put inside 'testplot' is 'TEST-PILOT'.
Can you help me to learn more?
(Other definitions for test pilot that I've seen before include "Highly dangerous job" , "Experimental flyer" , "high flier" , "one examining details of flight, spin, etc" , "Stop title (anag.)" .)