Promise to support development of what will enshrine Conservative slogan (9)
I believe the answer is:
watchword
'slogan' is the definition.
(I've seen this before)
'promise to support development of what will enshrine conservative' is the wordplay.
'promise' becomes 'word' (as in 'I give you my word').
'to support' says to put letters next to each other (in a down clue, some letters hold others up).
'development of' is an anagram indicator (I've seen 'development' mean this).
'will enshrine' is an insertion indicator.
'conservative' becomes 'c' (abbreviation commonly used in UK politics).
'what' is an anagram of 'wath'.
'wath' going around 'c' is 'watch'.
'word' put after 'watch' is 'WATCHWORD'.
(Other definitions for watchword that I've seen before include "rallying-cry" , "Slogan" .)