What holds oil left in glacial formation (7)
I believe the answer is:
drumlin
'glacial formation' is the definition.
(type of mound formed by a glacier)
'what holds oil left in' is the wordplay.
'what holds oil' becomes 'drum' (a drum is a container for oil).
'left' becomes 'l' (common abbreviation).
'drum'+'l'+'in'='DRUMLIN'
(Other definitions for drumlin that I've seen before include "'Small ridge, common in Cavan and Monaghan (7)'" , "Small elongated oval hill, in Cavan, say" , "oval mound" , "Hummock of boulder clay left by a retreating glacier" , "Small hill, common in Cavan and Monaghan" .)