One managing river dock on limitless river (9)
I believe the answer is:
executive
'one managing' is the definition.
(thesaurus)
'river dock on limitless river' is the wordplay.
'river' becomes 'exe' (River Exe).
'dock' becomes 'cut'.
'on' means one lot of letters go next to another.
'limitless' means to look at the middle letters.
The middle of 'river' is 'ive'.
'exe'+'cut'+'ive'='EXECUTIVE'
(Other definitions for executive that I've seen before include "Administrator, manager" , "Person who makes and carries out decisions" , "Branch of government charged with putting laws, agreements etc into effect" , "Having the power to act" , "Senior businessperson" .)