Mortar there exploded causing movement underfoot (5,6)
I believe the answer is:
earth tremor
'movement underfoot' is the definition.
(tremor beneath one's feet)
'mortar there exploded' is the wordplay.
'exploded' is an anagram indicator.
'mortar'+'there'='mortarthere'
'mortarthere' anagrammed gives 'EARTH TREMOR'.
'causing' is the link.
(Other definitions for earth tremor that I've seen before include "Slight quake" , "Minor quake" , "unsettling experience" , "Shock" .)