Soldiers only destroyed trust (4,2)
I believe the answer is:
'trust' is the definition.
(relying on is a kind of trusting)
'soldiers only destroyed' is the wordplay.
'soldiers' becomes 're' (Royal Engineers).
'destroyed' indicates anagramming the letters.
'only' with letters rearranged gives 'lyon'.
(Other definitions for rely on that I've seen before include "Trust in with confidence" , "Trust, be confident in" , "Put trust in" .)