Pen extremely legible type (5)
I believe the answer is:
style
'type' is the definition.
(style can mean a type or variety of something)
'pen extremely legible' is the wordplay.
'pen' becomes 'sty' (both can mean an animal enclosure).
'extremely' says to hollow out the word (remove centre letters) (only the extremes of the word are used).
'legible' with its middle removed is 'le'.
'sty'+'le'='STYLE'
(Other definitions for style that I've seen before include "Former writer" , "Method; elegance" , "Phraseology" , "Elan" , "Distinctive arrangement, elegance" .)