Lying Conservative in Number Ten endlessly partying (9)
I believe the answer is:
recumbent
'lying' is the definition.
(I've seen this before)
'conservative in number ten endlessly partying' is the wordplay.
'conservative' becomes 'C' (UK political abbreviation).
'in' means one lot of letters goes inside another.
'endlessly' means to remove the last letter.
'partying' is an anagram indicator (I've seen 'parties' mean this (the letters move or party around)).
'number'+'ten'='numberten'
'numberten' with its last letter taken away is 'numberte'.
'numberte' is an anagram of 'reumbent'.
'c' placed inside 'reumbent' is 'RECUMBENT'.
(Other definitions for recumbent that I've seen before include "Lying down comfortably" , "flat?" .)