Behind one's back? (6)
'behind' is the definition.
(I've seen this before)
'one's back?' is the wordplay.
'one' becomes 'a'.
'back?' becomes 'stern' (stern is a kind of back).
'a'+'stern'='ASTERN'
(Other definitions for astern that I've seen before include "Behind (a boat)" , "Toward the tail of a vessel" , "at the vessel's rear" , "Backward" , "Aft" .)
But I've got no explanation for this one! Could you help me understand?
(Other definitions for second that I've seen before include "Short time - back" , "Assistant in the ring" , "Time for a supporter" , "Instant - support" , "Lower < back" .)