Ponder over trace of ergot on popular plant (7)
I believe the answer is:
mullein
'plant' is the definition.
(mullein is a kind of plant)
'ponder over trace of ergot on popular' is the wordplay.
'ponder' becomes 'mull' (synonyms).
'over' means one lot of letters go next to another.
'trace of' suggests taking the first letters.
'on' says to put letters next to each other.
'popular' becomes 'in' ('in' can mean trendy).
The first letter of 'ergot' is 'e'.
'mull'+'e'+'in'='MULLEIN'
(Other definitions for mullein that I've seen before include "Plant with spikes of yellow flowers" , "Plant with woolly leaves" , "Yellow-flowered plant popularly known as Aaron's rod" .)