Welsh army heartlessly set ambush (6)
I believe the answer is:
waylay
'ambush' is the definition.
(synonyms)
'welsh army heartlessly set' is the wordplay.
'welsh' becomes 'W' (abbreviation).
'heartlessly' suggests removing the centre.
'set' becomes 'lay' (both can mean to place something).
'army' with its middle taken out is 'ay'.
'w'+'ay'+'lay'='WAYLAY'
(Other definitions for waylay that I've seen before include "Lie in wait for and ambush" , "Intercept and attack" , "Lie in wait for, to talk or rob (6)" , "Lie in wait for, maybe to rob" , "buttonhole" .)