Writer is upset covering the end of Joe Root (4)
I believe the answer is:
neep
'root' is the definition.
(I know that neep is a type of root vegetable)
'writer is upset covering the end of joe' is the wordplay.
'writer' becomes 'pen' (something which writes).
'is upset' says the letters should be written in reverse (I've seen 'being upset' mean this).
'covering' is an insertion indicator.
'the end of' suggests the final letters.
The last letter of 'joe' is 'e'.
'pen' in reverse letter order is 'nep'.
'nep' going around 'e' is 'NEEP'.
(Other definitions for neep that I've seen before include "Scots veg" , "Root" , "vegetable in Scotland" , "Scots turnip" , "(Scottish) turnip" .)