Days with siestas regularly — Royal Marines lay down weapons (6)
I believe the answer is:
disarm
'lay down weapons' is the definition.
(to disarm is to remove weapons)
'days with siestas regularly royal marines' is the wordplay.
'days' becomes 'd' (abbreviation for days).
'with' is a charade indicator (letters next to each other).
'regularly' indicates alternate letters (regularly take one letter, leave next etc.).
'royal marines' becomes 'RM' (abbreviation).
The alternating letters of 'siestas' are 'isa'.
'd'+'isa'+'rm'='DISARM'
(Other definitions for disarm that I've seen before include "Take away weapons or make less angry" , "Demilitarise" , "Confiscate weapon or mollify" , "Win the confidence (of)" , "Make less hostile, win over" .)