It’s unusual way to extend, and count, old measure (7,2,4)

'it's unusual' is the definition.
(phrase said before relating something unusual)

'way to extend and count old measure' is the wordplay.
I cannot really see how this works, but
'way' could be 'st' (abbreviation for street) and 'st' is found within the answer.
'to' is present in the answer.
'extend' could be 'range' (range can mean to extend or cover) and 'range' is present in the answer.
'count' could be 'tell' (I've seen this before) and 'tell' is present in the answer.
This accounts for all the letters.
This explanation may well be incorrect...