Follow Geoff A. Thompson inside (6)
I believe the answer is:
fathom
'follow' is the definition.
(thesaurus)
'geoff a thompson inside' is the wordplay.
'inside' indicates a hidden word.
'FATHOM' can be found hidden inside 'geoff a thompson'.
(Other definitions for fathom that I've seen before include "Sea depth of six feet" , "''Full ... five, thy father lies'' (The Tempest)" , "Comprehend - a depth of six feet" , "Understand (something) after much thought" , "Discover the meaning of - six feet" .)