Train sailor to grasp gallons and miles in Arabic maths (8)
I believe the answer is:
algorism
'arabic maths' is the definition.
(term for the system of Arabic numerals)
'train sailor to grasp gallons and miles' is the wordplay.
'train' is an anagram indicator (to train can mean to grow something into a shape).
'to grasp' means one lot of letters goes inside another.
'gallons' becomes 'g' ().
'and' is a charade indicator (letters next to each other).
'miles' becomes 'm' (common abbreviation).
'sailor' anagrammed gives 'aloris'.
'aloris' enclosing 'g' is 'algoris'.
'algoris'+'m'='ALGORISM'
'in' acts as a link.
(Another definition for algorism that I've seen is " arithmetic".)