Look hard, one side to the other, for weeds (5)
I believe the answer is:
tares
'look hard one side to the other for weeds' is the definition.
I can't tell whether this definition defines the answer.
'one side to the other for weeds' is the wordplay.
'one side' suggests halving the letters.
'the other for weeds' becomes 'ares' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'to' cut in half is 't'.
't'+'ares'='TARES'
Can you help me to learn more?
(Other definitions for tares that I've seen before include "Stare at the vetches" , "The weights of packaging" .)