Abbey not quite so upright, receiving onset of lightning strike (4,5)

'strike' is the definition.
(to down tools is to stop working and strike)

'abbey not quite so upright receiving onset of lightning' is the wordplay.
'abbey' becomes 'downton' (abbey in TV's Downton Abbey).
'not quite' means to remove the last letter.
'upright' says the letters should be written in reverse (in a down clue, letters go up).
'receiving' is an insertion indicator.
'onset of' suggests taking the first letters.
The first letter of 'lightning' is 'l'.
'so' written backwards gives 'os'.
'downton' with its final letter taken off is 'downto'.
'os' going around 'l' is 'ols'.
'downto'+'ols'='DOWN TOOLS'