It stretches, expands, primarily — first off, flexible (7)
I believe the answer is:
elastic
This clue could be a double definition.
'it stretches expands primarily first off' is the first definition.
The definition suggests a singular noun which matches the answer.
'flexible' is the second definition.
(associated in meaning)
Can you help me to learn more?
(Other definitions for elastic that I've seen before include "Like some bands" , "Flexible, adaptable" , "Something stretchy" , "It's lace (anag.)" , "Giving" .)