Frank and Jack, heading off around one (5)
I believe the answer is:
naive
'frank' is the definition.
(thesaurus)
'jack heading off around one' is the wordplay.
'jack' becomes 'knave' (knave can mean a jack playing card).
'heading off' means to remove the first letter.
'around' is an insertion indicator.
'one' becomes 'I' (Roman numeral).
'knave' with its initial letter removed is 'nave'.
'nave' going around 'i' is 'NAIVE'.
'and' acts as a link.
(Other definitions for naive that I've seen before include "Simple-minded" , "Too trusting and inexperienced" , "Ingenuous, easily deceived" , "Inexperienced or gullible" , "Lacking judgment" .)