Not unusually, one may be out about a foot (5)
I believe the answer is:
often
'about a foot' is the definition.
I can't judge whether this defines the answer.
'not unusually one may be out' is the wordplay.
'not unusually' becomes 'ft' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'may be' is an anagram indicator.
'out' is an insertion indicator.
'one' anagrammed gives 'oen'.
'ft' inserted into 'oen' is 'OFTEN'.
Can you help me to learn more?
(Other definitions for often that I've seen before include "Multiply" , "More than occasionally" , "on numerous occasions" , "Frequently, not seldom" , "A lot" .)