Big external dimension (7)
I believe the answer is:
outsize
'big' is the definition.
(similar in meaning)
'external dimension' is the wordplay.
'external' becomes 'out' (similar in meaning).
'dimension' becomes 'size' (I've seen this before).
'out'+'size'='OUTSIZE'
(Other definitions for outsize that I've seen before include "Extra large (clothes)" , "big girl's blouse, for example?" , "Much larger than average, say clothes" , "Exceptionally large" , "Extra-large (garment)" .)