Trendy lawyer's to sum up (2,5)
I believe the answer is:
in brief
'sum up' is the definition.
(I've seen this before)
'trendy lawyer's' is the wordplay.
'trendy' becomes 'in'.
'lawyer' becomes 'brief' (I've seen this before).
'in'+'brief'='IN BRIEF'
'to' is the link.
(Other definitions for in brief that I've seen before include "ungarnished?" , "shortly" , "Where barrister finds the facts" , "skip a lot" , "Where the solicitor should look for advice" .)