Mathematician once picked up in tanker (5)
I believe the answer is:
oiler
'tanker' is the definition.
(I know that tanker can be written as oiler)
'mathematician once picked up' is the wordplay.
'mathematician once' becomes 'Euler' (mathematician Leonhard Euler, 1707-1783).
'picked up' indicates a 'sounds like' (homophone) clue.
'euler' is a homophone of 'OILER'.
'in' is the link.
(Other definitions for oiler that I've seen before include "Fuel tanker" , "Grease monkey" , "Chippendales assistant?" , "Loire (anag.)" , "Ship" .)