Old call turns stomach: "At last, charge over the top!" (7)
I believe the answer is:
extreme
'over the top' is the definition.
(I've seen this before)
'old call turns stomach at last charge' is the wordplay.
I cannot really see how this works, but
'old' could be 'ex' (prefix meaning former or onetime) and 'ex' is found within the answer.
'last' could be 't' (I've seen this in other clues) and 't' is present in the answer.
This explanation may well be incorrect...
(Other definitions for extreme that I've seen before include "Greatest or smallest" , "Of the greatest intensity" , "Anything but moderate" , "Last" , "too much" .)