I must accompany soldiers in specialist trial (10)
I believe the answer is:
experiment
'trial' is the definition.
(synonyms)
'i must accompany soldiers in specialist' is the wordplay.
'must accompany' becomes 'expe' (I can't justify this - if you can you should give a lot more credence to this answer).
'soldiers' becomes 'RM' (Royal Marines).
'in' means one lot of letters goes inside another.
'specialist' becomes 'ent' (Ear, Nose and Throat specialist).
'expe'+'rm'='experm'
'i' placed within 'experm' is 'experim'.
'experim'+'ent'='EXPERIMENT'
Can you help me to learn more?
(Other definitions for experiment that I've seen before include "scientific investigation" , "Controlled test" , "Trial" .)