Rat steered off, heading for rathole (8)
I believe the answer is:
deserter
'rat' is the definition.
(thesaurus)
'steered off heading for rathole' is the wordplay.
'off' indicates an anagram.
'heading for' suggests taking the first letters.
The first letter of 'rathole' is 'r'.
'steered' anagrammed gives 'deserte'.
'deserte'+'r'='DESERTER'
(Other definitions for deserter that I've seen before include "Illegal army leaver" , "One illegally leaving armed forces" , "military offender" , "Turncoat" , "Defector" .)