Secret agent skins up with little sign of hesitation (7)
I believe the answer is:
sleeper
'secret agent' is the definition.
(I know that sleeper is a type of spy)
'skins up with little sign of hesitation' is the wordplay.
'skins' becomes 'peels' ('peel' can be a synonym of 'skin').
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
'with' is a charade indicator (letters next to each other).
'little sign of hesitation' becomes 'er' (short exclamation indicating hesitation).
'peels' reversed gives 'sleep'.
'sleep'+'er'='SLEEPER'
(Other definitions for sleeper that I've seen before include "Rail support; enemy spy" , "Form of transport" , "film not successful at first" , "One who's out" , "Kipper" .)