Spotted deer unknown in Asia, mostly running (4)
I believe the answer is:
axis
'spotted deer' is the definition.
(I've seen this before)
'unknown in asia mostly running' is the wordplay.
'unknown' becomes 'x' (algebra).
'in' indicates putting letters inside.
'mostly' means to remove the last letter (most but not all of the word).
'running' indicates an anagram (the letters run around to different places).
'asia' with its last letter taken off is 'asi'.
'asi' anagrammed gives 'ais'.
'x' inserted inside 'ais' is 'AXIS'.
(Other definitions for axis that I've seen before include "WW2 Nazi alliance" , "Imaginary central line" , "Imaginary line about which a body rotates and a Dublin theatre" , "1936 alliance between Germany and Italy" , "where wheel turns?" .)