Swell, once, a leader of Tories, eclipsed by another politician (7)

'swell once' is the definition.
(greaten could historically mean to swell)

'a leader of tories eclipsed by another politician' is the wordplay.
'leader of' indicates taking the first letters.
'eclipsed by' indicates putting letters inside.
'another politician' becomes 'green' (eg member of the Green Party. I am not sure about the 'another' bit.).
The first letter of 'tories' is 't'.
'a'+'t'='at'
'at' going within 'green' is 'GREATEN'.