Daft as one student into Stallone (5)
I believe the answer is:
silly
'daft' is the definition.
(I've seen this before)
'one student into stallone' is the wordplay.
'one' becomes 'i' (Roman numeral).
'student' becomes 'l' (as in L-plates for learner drivers).
'into' indicates putting letters inside.
'stallone' becomes 'sly' (nickname for Sylvester Stallone).
'i'+'l'='il'
'il' put inside 'sly' is 'SILLY'.
'as' acts as a link.
(Other definitions for silly that I've seen before include "Idiotic" , "near the batsman" , "Preposterous" , "Daft" , "Stupid" .)