Gather work engrosses old politicians (5,2)
I believe the answer is:
round up
'gather' is the definition.
(rounding up is a kind of gathering)
'work engrosses old politicians' is the wordplay.
'work' becomes 'run' (both can mean to operate something**).
'engrosses' means one lot of letters goes inside another.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'politicians' becomes 'dup' (Democratic Unionist Party).
'run' enclosing 'o' is 'roun'.
'roun'+'dup'='ROUND-UP'
(Other definitions for round up that I've seen before include "marginally ahead in boxing bout?" , "herd" , "Use less accurate figure" , "For convenience, increase the number" , "collection of data" .)