Talk up front in hall on variable exercises (4)
I believe the answer is:
hype
'talk up' is the definition.
(I've seen this in another clue)
'front in hall on variable exercises' is the wordplay.
'front in' suggests taking the first letters (I've seen 'fronts in' mean this (the letters are the front)).
'on' says to put letters next to each other.
'variable' becomes 'y' (y may represent a variable in algebra).
'exercises' becomes 'pe' (Physical Education class).
The first letter of 'hall' is 'h'.
'h'+'y'+'pe'='HYPE'
(Other definitions for hype that I've seen before include "Advert" , "Doer of dirty work" , "Publicity stunt" , "Puffery" , "Agitate" .)