Crew's first pair in U-boat clean deck thus (5)
I believe the answer is:
scrub
'clean deck thus' is the definition.
I can't judge whether this definition defines the answer.
'crew's first pair in u-boat' is the wordplay.
'crew's first' becomes 'c' (rst letter of 'crew').
'pair' becomes 'r' (I am not sure about this - if you are sure you should believe this answer much more).
'in' means one lot of letters goes inside another.
'u-boat' becomes 'sub' (German submarine).
'c'+'r'='cr'
'cr' going inside 'sub' is 'SCRUB'.
Can you help me to learn more?
(Other definitions for scrub that I've seen before include "Cancel; clean vigorously" , "Vegetation consisting of stunted trees or bushes" , "Axe" , "Scour; cancel" , "Powerful wash" .)