Ceasefire, short, set up on base (5)
I believe the answer is:
truce
'ceasefire' is the definition.
(both can mean a break in fighting)
'short set up on base' is the wordplay.
'short' becomes 'curt' (synonyms).
'set up' says the letters should be written in reverse (in a down clue, letters go up).
'on base' becomes 'e' (Euler's number - base of natural logarithms. I am not sure about the 'on' bit.).
'curt' reversed gives 'truc'.
'truc'+'e'='TRUCE'
Can you help me to learn more?
(Other definitions for truce that I've seen before include "Respite" , "Agreement to suspend fighting" , "Temporary cessation of hostilities" , "conciliatory agreement" , "Agreed ceasefire" .)