It may grow in a tub with some herbs (5)
I believe the answer is:
tuber
'it may grow' is the definition.
The definition suggests a singular noun which matches the answer.
'a tub with some herbs' is the wordplay.
'with some herbs' becomes 'er' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'tub'+'er'='TUBER'
'in' is the link.
Can you help me to learn more?
(Other definitions for tuber that I've seen before include "Fleshy root, vegetable say" , "Type of plant, potato say" , "Fleshy underground part of a stem, often used as food" , "Potato maybe" , "traveller down in London?" .)