Proportion having a time in port (5)
I believe the answer is:
ratio
'proportion' is the definition.
(thesaurus)
'a time in port' is the wordplay.
'a time' becomes 'at' (I am not sure about this - if you are sure you should believe this answer much more).
'in' indicates putting letters inside.
'port' becomes 'rio' (I've seen this before).
'at' put inside 'rio' is 'RATIO'.
'having' acts as a link.
Can you help me to learn more?
(Other definitions for ratio that I've seen before include "Eg, 4 to 3" , "Comparative figure" , "Measure expressed as a proportion" , "Comparative measure" , "Relative magnitudes of two quantities" .)