Royal Engineers said to go free (7)
I believe the answer is:
relieve
'go free' is the definition.
'relieve' can be an answer for 'free' (I've seen this before). I'm not certain of the 'go' bit.
'royal engineers said' is the wordplay.
'royal engineers' becomes 're'.
'said' becomes 'lieve' (I can't explain this - if you can you should give a lot more credence to this answer).
're'+'lieve'='RELIEVE'
'to' acts as a link.
Can you help me to learn more?
(Other definitions for relieve that I've seen before include "Take place of" , "Alleviate pain or stress" , "raise a siege" , "Alleviate (eg, pain)" , "Set free" .)