Beer mostly fills half a dozen small bottles (5)
I believe the answer is:
vials
'bottles' is the definition.
(vial is a kind of bottle)
'beer mostly fills half a dozen small' is the wordplay.
'beer' becomes 'ale' (I've seen this before).
'mostly' means to remove the last letter.
'fills' is an insertion indicator.
'half a dozen' becomes 'vi' ('vi' can be a synonym of 'half a dozen').
'small' becomes 's' (abbreviation - e.g. clothes size).
'ale' with its final letter taken away is 'al'.
'vi'+'s'='vis'
'al' placed into 'vis' is 'VIALS'.
(Other definitions for vials that I've seen before include "Small bottle that contains a drug" , "Vessels" , "Small bottles of medicine" , "Small glass containers" , "Ampoules" .)