Definitely how a thought may be acted out (6)
I believe the answer is:
indeed
'definitely' is the definition.
(thesaurus)
I cannot understand how the remainder of the clue works.
(Other definitions for indeed that I've seen before include "Really - what a surprise!" , "still" , "In truth, in fact" , "Word expressing surprise, disbelief, etc" , "Really, without doubt" .)