Liberal politician, Welsh, not quite in senior position (4)
I believe the answer is:
whig
'liberal politician' is the definition.
'welsh not quite in senior position' is the wordplay.
'welsh' becomes 'w'.
'not quite' means to remove the last letter.
'in senior position' becomes 'high'.
'high' with its final letter taken off is 'hig'.
'w'+'hig'='WHIG'
(Other definitions for whig that I've seen before include "Former political party member" , "Former 14 Down" , "political operator" , "Antecedent of a Liberal politician in Britain" , "Supporter of an old English political party" .)