What could make mariner try to join up with relative? (10)
I believe the answer is:
intermarry
'to join up with relative?' is the definition.
'what could make mariner try' is the wordplay.
'what could make' indicates an anagram.
'mariner'+'try'='marinertry'
'marinertry' with letters rearranged gives 'INTERMARRY'.
(Other definitions for intermarry that I've seen before include "unite two races" , "Wed within family group" , "(Of groups, races) mingle through weddings" , "try unifying countries?" .)