Distress signal? Get on securing line (5)
I believe the answer is:
flare
'distress signal?' is the definition.
(I've seen this in another clue)
'get on securing line' is the wordplay.
'get on' becomes 'fare' (I've seen this in another clue).
'securing' means one lot of letters goes inside another.
'line' becomes 'l' (used when specifying particular lines from a poem).
'fare' enclosing 'l' is 'FLARE'.
(Other definitions for flare that I've seen before include "Feral kind of sudden activity" , "Blaze with sudden irregular flame" , "Widen out like a bell" , "Spread" , "Sudden burst of flame" .)