Navy occupying vast river that's long (6)
I believe the answer is:
hunger
'long' is the definition.
(thesaurus)
'navy occupying vast river' is the wordplay.
'navy' becomes 'N' (common abbreviation eg in RN Royal Navy).
'occupying' is an insertion indicator (inserted letters occupy or enter the word).
'vast' becomes 'huge' (I've seen this in another clue).
'river' becomes 'r' (abbreviation for river).
'n' placed inside 'huge' is 'hunge'.
'hunge'+'r'='HUNGER'
'that's' is the link.
(Other definitions for hunger that I've seen before include "Strong desire - sort of strike?" , "Strong desire for food" , "more like a divided jury?" , "Longing for food" , "Tummy-rumbling desire" .)