Worried about small vessels (1-5)
I believe the answer is:
u-boats
'vessels' is the definition.
(U-boat is an example)
'worried about small' is the wordplay.
'worried' indicates anagramming the letters (worry can mean to interfere with or disturb).
'small' becomes 's' (abbreviation - e.g. clothes size).
'about' with letters rearranged gives 'uboat'.
'uboat'+'s'='U-BOATS'
(Other definitions for u-boats that I've seen before include "German submarines in WW2" , "Submersible warships" , "German submarines of second World War" , "Submersible ships of WW2" , "German subs" .)