Large increase definitely — about 1000 (5)
I believe the answer is:
surge
'large increase' is the definition.
'surge' can be an answer for 'increase' (I've seen this before). I am not sure about the 'large' bit.
'definitely about 1000' is the wordplay.
'definitely' becomes 'sure' (both can mean 'yes').
'about' is an insertion indicator.
'1000' becomes 'g' (G can mean a grand - 1000 pounds, dollars etc).
'sure' placed around 'g' is 'SURGE'.
Can you help me to learn more?
(Other definitions for surge that I've seen before include "Suddenly increase" , "Rapid increase; powerful movement" , "Sudden upward movement" , "Sounds as if a swell" , "Increase rapidly in intensity" .)