Thoroughly reliable policeman reached the bed (6,8)
I believe the answer is:
copper bottomed
'thoroughly reliable' is the definition.
'copper bottomed' can be an answer for 'reliable' (I've seen this before). I'm not sure about the 'thoroughly' bit.
'policeman reached the bed' is the wordplay.
'policeman' becomes 'copper' (copper is a kind of policeman).
'reached the bed' becomes 'bottomed' (I can't justify this - if you can you should give a lot more credence to this answer).
'copper'+'bottomed'='COPPER-BOTTOMED'
Can you help me to learn more?
(Other definitions for copper bottomed that I've seen before include "Very safe" , "being financially sound" , "regarded as reliable" .)