Face, regularly, Spain's German guards, and shoot back (4,5)
I believe the answer is:
sans serif
'face' is the definition.
(type of typeface)
'regularly spain's german guards and shoot back' is the wordplay.
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
'german guards' becomes 'SS' (Schutzstaffel, Nazi military force).
'and' is a charade indicator (letters next to each other).
'shoot' becomes 'fire' (as in firing a gun).
'back' says the letters should be written backwards.
The alternating letters of 'spains' are 'san'.
'fire' reversed gives 'erif'.
'san'+'ss'+'erif'='SANS SERIF'
(Other definitions for sans serif that I've seen before include "Plain typeface" , "Style of font" , "plain type of face" , "Plain style of type" , "Style of typeface" .)