Entertainment, initially on platform, before time (5)
I believe the answer is:
opera
'entertainment' is the definition.
(I've seen this before)
'initially on platform before time' is the wordplay.
'initially' says to take the initial letters.
'before' means one lot of letters go next to another.
'time' becomes 'era' (era is a kind of time).
The initial letters of 'on platform' is 'op'.
'op'+'era'='OPERA'
(Other definitions for opera that I've seen before include "Eg, La forza del destino" , ""Rigoletto", for example" , "Musical in which all the words are sung" , "La Scala production, perhaps" , "Carmen, perhaps" .)