Coalition barely able to be supported by old Conservative (4)
I believe the answer is:
bloc
'coalition' is the definition.
(I know that bloc is a type of coalition)
'barely able to be supported by old conservative' is the wordplay.
'barely' means to look at the middle letters (the word is bare of its external letters).
'to be supported by' means one lot of letters go next to another (in a down clue, some letters hold up others).
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'conservative' becomes 'C' (UK political abbreviation).
The central letters of 'able' are 'bl'.
'bl'+'o'+'c'='BLOC'
(Other definitions for bloc that I've seen before include "Allied group of countries or political parties" , "Group of allied nations" , "Group of parties or countries united in common interest - sounds wooden" , "A number of countries" , "International alliance" .)