Shorten a span (7)
I believe the answer is:
abridge
'shorten' is the definition.
('abridge' can be a synonym of 'shorten')
'a span' is the wordplay.
'span' becomes 'bridge' ('bridge' can be a synonym of 'span').
'a'+'bridge'='ABRIDGE'
(Other definitions for abridge that I've seen before include "Make shorter in brigade" , "prune?" , "Concentrate" , "Condense, summarise" , "Abbreviate, contract" .)