Bill will have to call back for this kind of acid (6)
I believe the answer is:
acetic
'this kind of acid' is the definition.
(I've seen this in another clue)
'bill will have to call back' is the wordplay.
'bill' becomes 'ac' (short for account).
'will have' says to put letters next to each other.
'to call' becomes 'cite' (citing is a kind of calling).
'back' says the letters should be written backwards.
'cite' back-to-front is 'etic'.
'ac'+'etic'='ACETIC'
'for' acts as a link.
(Other definitions for acetic that I've seen before include "really sour" , "Type of acid derived from vinegar" , "tart" , "Like vinegar's acid" , "Saucy'" .)