Amount of liquid woman, about 51, took regularly at first (10)
I believe the answer is:
millilitre
'amount of liquid' is the definition.
'woman about 51 took regularly at first' is the wordplay.
'woman' becomes 'millie' (**).
'about' is an insertion indicator.
'im' becomes 'li' (Roman numerals).
'at first' says to take the initial letters.
The initial letters of 'took regularly' is 'tr'.
'li'+'tr'='litr'
'millie' enclosing 'litr' is 'MILLILITRE'.
(Other definitions for millilitre that I've seen before include "Small measure of liquid" , "Cubic centimetre" .)