Engineer enlarges core of junction boxes (10)
I believe the answer is:
rectangles
'boxes' is the definition.
'engineer enlarges core of junction' is the wordplay.
'engineer' indicates an anagram.
'core of' means to look at the middle letters.
The middle of 'junction' is 'ct'.
'enlarges'+'ct'='enlargesct'
'enlargesct' is an anagram of 'RECTANGLES'.
(Other definitions for rectangles that I've seen before include "they may be squares" , "Four-sided figures" .)