Extend agreement to cover results over target range, initially (8)
I believe the answer is:
protract
'extend' is the definition.
('protract' can be a synonym of 'extend')
'agreement to cover results over target range initially' is the wordplay.
'agreement' becomes 'pact' (pact is a kind of agreement**).
'to cover' means one lot of letters goes inside another.
'initially' indicates taking the first letters.
The first letters of 'results over target range' is 'rotr'.
'pact' placed around 'rotr' is 'PROTRACT'.
(Other definitions for protract that I've seen before include "Increase" , "Drag out at length" , "Draw out, prolong" , "Extend length" , "Draw out, lengthen the time" .)