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15d, 2words, 5.4. G???? ?o??, Through bursary, departs on our long trip.
17a, S????, Was bather panicking at first in flood?.
18d, ????e??, Textbook's Iron Cross initially appearing in bold.
21a, U?????e?, Submarine manoeuvres ensured opening for Allies.
27a, S?????n?n, Something in blood half seen - not iron, unfortunately.

15d - grand tour

Thank you liz.

17a - SWAMP

Thanks, pigale.

It's a bit the blind leading the bliind here !

But for 27a I just tried to put SE (half of seen) with NOT IRON on anagram solver and it gave me
NEOSITRON - which is an anti-coagulant!

No idea if correct, so perhaps someone will confirm - for both of us !

Sorry, just realized it does not concur with the letters you have already -

27 Seratonin - SE + not iron anag

Serotonin sorry

18 Perfect Fe C in Pert

21 Undersea - Ensured + A anag

Thank you pigale , mamya for your help very grateful.