Inactivity shown by soldiers in busy port (6)
I believe the answer is:
torpor
'inactivity' is the definition.
(torpor is a kind of inactivity)
'soldiers in busy port' is the wordplay.
'soldiers' becomes 'or' (Other Rank).
'in' indicates putting letters inside.
'busy' indicates an anagram.
'port' with letters rearranged gives 'tpor'.
'or' going within 'tpor' is 'TORPOR'.
'shown by' acts as a link.
(Other definitions for torpor that I've seen before include "Sluggish and inactive state" , "Listlessness" , "Numbness - inactivity" , "State of sluggish somnolence" , "State of sluggishness and inactivity" .)