Sylvester Stallone's hiding at first (4)
I believe the answer is:
erst
'at first' is the definition.
(I've seen this before)
'sylvester stallone's hiding' is the wordplay.
'hiding' says the answer is hidden in the clue.
'ERST' can be found hidden inside 'sylvester stallone'.
(Other definitions for erst that I've seen before include "At first; formerly" , "Rest turned formerly" , "in the past" , "in former times" .)