Extremely tough grain, more than once (6)
I believe the answer is:
thrice
'than once' is the definition.
I can't judge whether this definition defines the answer.
'extremely tough grain more' is the wordplay.
'extremely' says to hollow out the word (remove centre letters).
'grain' becomes 'rice' (rice is a kind of grain).
'more' says to put letters next to each other (some letters added to the previous ones).
'tough' with its middle removed is 'th'.
'th'+'rice'='THRICE'
Can you help me to learn more?
(Other definitions for thrice that I've seen before include "Old usage for three times" , "On three occasions" , "In threefold degree" , "as it can be found in little titbits" , "Once, once more and yet once more" .)