Giant gnome gardener keeps inside (4)
I believe the answer is:
mega
'giant' is the definition.
(mega means very big)
'gnome gardener keeps inside' is the wordplay.
'inside' indicates a hidden word.
'MEGA' can be found hidden inside 'gnome gardener keeps'.
(Other definitions for mega that I've seen before include "wicked" , "Fantastic!" , "Enormous; very good" , "Huge (slang)" , "ace" .)