Quality of alien with soldiers clutching the head (9)
I believe the answer is:
otherness
'quality of alien' is the definition.
(quality of being different or alien)
'soldiers clutching the head' is the wordplay.
'soldiers' becomes 'or' (Other Rank).
'clutching' indicates putting letters inside.
'head' becomes 'ness' (ness is a headland).
'or' going around 'the' is 'other'.
'other'+'ness'='OTHERNESS'
'with' is the link.
(Other definitions for otherness that I've seen before include "Quality of being different" , "Distinctive quality" .)