One who chronicles onslaught repulsed first (7)
I believe the answer is:
diarist
'one who chronicles' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'onslaught repulsed first' is the wordplay.
'onslaught' becomes 'raid' (raid is a kind of onslaught).
'repulsed' shows that the letters should be reversed in order.
'first' becomes 'ist' (ist resembles 1st).
'raid' written backwards gives 'diar'.
'diar'+'ist'='DIARIST'
Can you help me to learn more?
(Other definitions for diarist that I've seen before include "Pepys, say" , "Frank, say" , "eg Samuel Pepys" , "e.g. John Evelyn" , "One as Evelyn, Kilvert" .)