Soldier with no idea, losing heart in irrational fear (8)
I believe the answer is:
paranoia
'irrational fear' is the definition.
(I've seen this in another clue)
'soldier with no idea losing heart' is the wordplay.
'soldier' becomes 'para' (short for paratrooper).
'with' says to put letters next to each other.
'losing heart' suggests removing the centre.
'idea' with its middle removed is 'ia'.
'para'+'no'+'ia'='PARANOIA'
'in' is the link.
(Other definitions for paranoia that I've seen before include "Disorder characterised by delusions of persecution or grandeur" , "Irrational fear or suspicion" , "Disorder characterized by delusions of persecution" , "Insane suspiciousness" , "Delusion of being persecuted" .)