Sailor back with one old relationship (5)
I believe the answer is:
ratio
'relationship' is the definition.
(thesaurus)
'sailor back with one old' is the wordplay.
'sailor' becomes 'tar' (tar is a kind of sailor**).
'back' is a reversal indicator.
'with' says to put letters next to each other.
'one' becomes 'i' (Roman numeral).
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'tar' back-to-front is 'rat'.
'rat'+'i'+'o'='RATIO'
(Other definitions for ratio that I've seen before include "Quotient, proportion" , "One-to-one, maybe" , "E.g. two to one" , "Relationship between two amounts" , "Proportional measure" .)