One sailor picked up old relation (5)
I believe the answer is:
ratio
'relation' is the definition.
(ratio is a kind of relation)
'one sailor picked up old' is the wordplay.
'one' becomes 'i' (Roman numeral).
'sailor' becomes 'tar' (slang term for a sailor).
'picked up' is a reversal indicator.
'old' becomes 'o' (abbreviation).
'i'+'tar'='itar'
'itar' backwards is 'rati'.
'rati'+'o'='RATIO'
(Other definitions for ratio that I've seen before include "Mathematical relation of one thing to another" , "Eg, three to one" , "Proportion of one thing to another" , "five to four, perhaps" , "Proportion, relation" .)