Study algebraic variables (I’m up in them) prepared for prompt use (5-3)
I believe the answer is:
ready-mix
'prepared for prompt use' is the definition.
'study algebraic variables i'm up in them' is the wordplay.
'study' becomes 'read' ('read' can be a synonym of 'study').
'algebraic variables' becomes 'yx' (x and y are variables in algebra).
'up' indicates anagramming the letters.
'in them' is an insertion indicator.
'im' anagrammed gives 'mi'.
'yx' enclosing 'mi' is 'ymix'.
'read'+'ymix'='READY-MIX'
(Other definitions for ready-mix that I've seen before include "Of food ingredients or concrete, prepared in advance" , "Pre-prepared, requiring only the addition of liquid" , "Easy-to-use concrete" , "Concrete quick to prepare" , "With ingredients already combined" .)