Gather Welsh politician’s a fool (5)
I believe the answer is:
amass
'gather' is the definition.
(synonyms)
'welsh politician's a fool' is the wordplay.
'welsh politician' becomes 'am' (I've seen this in other clues).
'a fool' becomes 'ass' (ass is a kind of fool).
'am'+'ass'='AMASS'
(Other definitions for amass that I've seen before include "store" , "Accumulate over time" , "Assemble" , "Gathers" , "Stock up on" .)