It has a stirring influence beneath the surface (12)
I believe the answer is:
undercurrent
'it has a stirring influence' is the definition.
The definition suggests a singular noun which matches the answer.
'beneath the surface' is the wordplay.
'beneath' becomes 'under' (under means beneath or underneath).
'the surface' becomes 'current' (I can't justify this - if you can you should believe this answer much more).
'under'+'current'='UNDERCURRENT'
Can you help me to learn more?
(Other definitions for undercurrent that I've seen before include "Subsurface influence" , "Hidden emotional feeling" , "Suppressed feeling" , "Emotion beneath apparent feeling" , "Sub-surface flow" .)