Jack is simply showing a light touch (4)
I believe the answer is:
kiss
'a light touch' is the definition.
(thesaurus)
'jack is simply showing' is the wordplay.
'showing' indicates a hidden word.
'KISS' is hidden within 'jack is simply'.
(Other definitions for kiss that I've seen before include "Embrace" , "Caress with lips" , "Peck (on cheek or lips?)" , "Touching of lips" , "Act of osculation" .)