Subs turning to a sub (1-5)
I believe the answer is:
u-boats
'subs' is the definition.
('U-boat' can be a synonym of 'sub')
'turning to a sub' is the wordplay.
'turning' indicates an anagram.
'to'+'a'+'sub'='toasub'
'toasub' anagrammed gives 'U-BOATS'.
(Other definitions for u-boats that I've seen before include "Submersible warships" , "German subs" , "German submarines in WW2" , "German submarines of second World War" , "Submersible ships of WW2" .)