One who shot his bolt taking lines and acid originally is very revealing (4-3)

'very revealing' is the definition.
(something tell-all is sensationally revealing)

'one who shot his bolt taking lines and acid originally' is the wordplay.
'one who shot his bolt' becomes 'Tell' (folk hero William Tell shot a crossbrow).
'taking' means one lot of letters go next to another.
'lines' becomes 'll' (I've seen this in other clues).
'and' means one lot of letters go next to another.
'originally' says to take the initial letters.
The first letter of 'acid' is 'a'.
'll' after 'a' is 'all'.
'tell'+'all' is 'TELL ALL'.

'is' acts as a link.