Soldiers free to relax with spliff (6)
I believe the answer is:
reefer
'spliff' is the definition.
(I've seen this before)
'soldiers free to relax' is the wordplay.
'soldiers' becomes 're' (Royal Engineers).
'to relax' indicates an anagram (I've seen 'relaxes' mean this).
'free' with letters rearranged gives 'efer'.
're'+'efer'='REEFER'
'with' is the link.
(Other definitions for reefer that I've seen before include "packed with cannabis?" , "Warm jacket - cannabis cigarette" , "Jacket for a sailor" , "Canvas trimmer" , "Cigarette containing cannabis - jacket" .)