OK, back around 1.50 with a mass unit (4)
I believe the answer is:
kilo
'a mass unit' is the definition.
'kilo' can be an answer for 'unit' (kilo is a kind of unit). I am not sure about the 'a mass' bit.
'ok back around 1 50' is the wordplay.
'back' says the letters should be written backwards.
'around' is an insertion indicator.
'l' becomes 'I' (roman numeral).
'50' becomes 'l' (roman numeral).
'ok' backwards is 'ko'.
'i'+'l'='il'
'ko' placed around 'il' is 'KILO'.
'with' is the link.
Can you help me to learn more?
(Other definitions for kilo that I've seen before include "Prefix for thousand" , "Metric unit of weight" , "Metrical unit of weight" , "Unit of mass" , "Measure of weight" .)