Measure of capacity or very old measure of light, mostly (6)
I believe the answer is:
volume
'measure of capacity' is the definition.
'very old measure of light mostly' is the wordplay.
'very' becomes 'v' (abbreviation).
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'measure of light' becomes 'lumen' (scientific unit).
'mostly' means to remove the last letter.
'lumen' with its final letter removed is 'lume'.
'v'+'o'+'lume'='VOLUME'
'or' is the link.
(Other definitions for volume that I've seen before include "Book; capacity" , "reading matter?" , "Magnitude of sound" , "Intensity of sound, in a book?" , "Loudness of sound" .)