Tons invested in visionary guide (5)
I believe the answer is:
steer
'guide' is the definition.
(I know that guide can be written as steer)
'tons invested in visionary' is the wordplay.
'tons' becomes 't'.
'invested in' indicates putting letters inside.
'visionary' becomes 'seer' (synonyms).
't' placed into 'seer' is 'STEER'.
(Other definitions for steer that I've seen before include "member of herd" , "Reset so as to direct" , "Manoeuvre" , "Drive; control" , "bovine individual?" .)