Surprise a lot when weight is heavy at the outset (8)
I believe the answer is:
astonish
'surprise a lot' is the definition.
(to astonish is to surprise very much)
'when weight is heavy at the outset' is the wordplay.
'when' becomes 'as' (eg 'when I walked' means 'as I walked').
'weight' becomes 'ton' (unit of weight).
'at the outset' indicates taking the first letters (the outset is the beginning).
The initial letter of 'heavy' is 'h'.
'as'+'ton'+'is'+'h'='ASTONISH'
(Other definitions for astonish that I've seen before include "'knock for six'?" , "are staggering" , "Amaze, astound" , "Stun" , "Startle" .)