Amount of data Rab found on Yankee kept by French head (8)
I believe the answer is:
terabyte
'amount of data' is the definition.
'rab found on yankee kept by french head' is the wordplay.
'found on' means one lot of letters go next to another.
'yankee' becomes 'y' (phonetic alphabet: alpha, bravo, charlie etc.).
'kept by' is an insertion indicator.
'french head' becomes 'tete' ('head' in French).
'rab'+'y'='raby'
'raby' placed within 'tete' is 'TERABYTE'.
(Other definitions for terabyte that I've seen before include "Large unit of computer memory" , "1000 gigabytes of data information" .)