Bill had to change into gear giving Charlie a head start (6)
I believe the answer is:
charge
'bill' is the definition.
(I know that bill can be written as charge)
'to change into gear giving charlie a head start' is the wordplay.
'to change into' is an anagram indicator.
'giving' says to put letters next to each other.
'start' suggests taking the first letters.
The initial letters of 'charlie head' is 'ch'.
'gear'+'ch'='gearch'
'gearch' anagrammed gives 'CHARGE'.
'had' acts as a link.
(Other definitions for charge that I've seen before include "Price asked; fill" , "Attack - require payment" , "impeachment" , "Care" , "In which an offender may be given" .)