Tax overruns on the plant (5)
I believe the answer is:
cress
'the plant' is the definition.
(cress is a kind of plant)
'tax overruns on' is the wordplay.
I cannot quite see how this works, but
'on' could be 're' ('on' can be mean 'with reference to') and 're' is found in the answer.
This may be the basis of clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for cress that I've seen before include "Plant with pungent leaves used in salads" , "Green cold salad ingredient" , "Toll of bell" , "Salad sprouts" , "Plant with leaves used in salad or as garnish" .)