Soldier, one with a fixation (5)
I believe the answer is:
mania
'fixation' is the definition.
(thesaurus)
'soldier one with a' is the wordplay.
'soldier' becomes 'man' (I've seen this before**).
'one' becomes 'i' (Roman numeral).
'with' says to put letters next to each other.
'man'+'i'+'a'='MANIA'
(Other definitions for mania that I've seen before include "Frenzy" , "Aberration" , "A (violent) mood disorder" , "Madness - craze" , "Obsessional enthusiasm" .)