Combined fighting force starts to experience difficulties (6)
I believe the answer is:
united
'combined' is the definition.
('unite' can be a synonym of 'combine')
'fighting force starts to experience difficulties' is the wordplay.
'fighting force' becomes 'unit' (military unit).
'starts to' suggests taking the first letters.
The initial letters of 'experience difficulties' is 'ed'.
'unit'+'ed'='UNITED'
(Other definitions for united that I've seen before include "Joined in marriage" , "Joined - football club" , "Of one mind" , "what may be seen after Oxford and Cambridge?" , "Allied, standing together" .)