Tick or similar critter biting Greek character (6)
I believe the answer is:
minute
'tick' is the definition.
(thesaurus)
'similar critter biting greek character' is the wordplay.
'similar critter' becomes 'mite' (I can't explain this - if you can you should give a lot more credence to this answer).
'biting' is an insertion indicator (some letters bite or swallow others).
'greek character' becomes 'nu' (letter from the Greek alphabet).
'mite' enclosing 'nu' is 'MINUTE'.
'or' acts as a link.
Can you help me to learn more?
(Other definitions for minute that I've seen before include "Formal note" , "It's not very long" , "Very small; record" , "Sixty seconds" , "Small amount of time" .)