Electric eel shows reserve (3)
I believe the answer is:
ice
'reserve' is the definition.
(both can mean aloofness)
'electric eel shows' is the wordplay.
'shows' indicates a hidden word.
'ICE' is hidden in the letters of 'electric eel'.
(Other definitions for ice that I've seen before include "Finish decorating the Christmas cake" , "Diamonds (informal)" , "Do cake decoration" , "Solid H2O" , "Block" .)