Walloping in secret? (6)
I believe the answer is:
hiding
I believe this is a double definition.
'walloping' is the first definition.
(thesaurus)
'secret?' is the second definition.
(I've seen this before)
'in' acts as a link.
(Other definitions for hiding that I've seen before include "Place of concealment" , "Beating; keeping out of sight" , "Hushing up" , "Evading capture," , "Suppressing" .)