Saltpetre starts to nestle in the reclining end (5)
I believe the answer is:
nitre
'saltpetre' is the definition.
(I know that saltpetre can be written as nitre)
'starts to nestle in the reclining end' is the wordplay.
'starts to' says to take the initial letters.
The initial letters of 'nestle in the reclining end' is 'NITRE'.
(Other definitions for nitre that I've seen before include "explosive ingredient" , "explosive producer?" , "Sodium nitrate" , "Chemical" , "Mineral used in gunpowder" .)