Lots of dogs head off (6)
I believe the answer is:
oodles
'lots of' is the definition.
(I've seen this before)
'dogs head off' is the wordplay.
'dogs' becomes 'poodles' (poodle is a kind of dog).
'head off' means to remove the first letter.
'poodles' with its first letter removed is 'OODLES'.
(Other definitions for oodles that I've seen before include "Plenty" , "A great many" , "Tons, scads" , "Large number or amount" , "masses" .)