Break cover say with corps retreating (6)
I believe the answer is:
emerge
'break cover' is the definition.
(thesaurus)
'say with corps retreating' is the wordplay.
'say' becomes 'eg' (both can mean 'for example').
'with' says to put letters next to each other.
'corps' becomes 'reme' (Royal Electrical and Mechanical Engineers).
'retreating' says the letters should be written backwards.
'eg'+'reme'='egreme'
'egreme' back-to-front is 'EMERGE'.
(Other definitions for emerge that I've seen before include "Develop" , "Move out and become visible" , "Become gradually apparent" , "Materialize" , "Come out or become known" .)