Sappers taking seconds to destroy train again (7)

'train again' is the definition.
(to reskill is to re-train with new skills)

'sappers taking seconds to destroy' is the wordplay.
I cannot quite see how this works, but
'sappers' could be 're' (Royal Engineers) and 're' is located in the answer.
'seconds' could be 's' and 's' is present in the answer.
'destroy' could be 'kill' (to kill is to destroy) and 'kill' is present in the answer.
This accounts for all the letters.
This may be the basis of clue (or it may be nonsense).