Stretch one's mind with centre for reading (7)
I believe the answer is:
distend
'stretch' is the definition.
(I've seen this before)
'one's mind with centre for reading' is the wordplay.
'one's' becomes 'i's' (Roman numeral).
'mind' becomes 'tend' (to tend is to look after or mind somebody).
'with' means one lot of letters go next to another.
'centre for' means to look at the middle letters.
The middle of 'reading' is 'd'.
'is'+'tend'='istend'
'istend' put after 'd' is 'DISTEND'.
(Other definitions for distend that I've seen before include "blow up" , "Make you bulge" , "balloon" , "Stretch" , "Bloat" .)