Teacher claiming European sailor discovered odd marine creature (7)
I believe the answer is:
seabird
'marine creature' is the definition.
(I've seen this before)
'teacher claiming european sailor discovered odd' is the wordplay.
'teacher' becomes 'sir' (a male teacher might be addressed as 'sir').
'claiming' indicates putting letters inside.
'european' becomes 'E' (abbreviation e.g. EU).
'sailor' becomes 'ab' (abbreviation for able seaman).
'discovered' means to look at the middle letters (dis-covered - outer letters removed).
The middle letter of 'odd' is 'd'.
'e'+'ab'='eab'
'sir' enclosing 'eab' is 'seabir'.
'seabir'+'d'='SEABIRD'
(Other definitions for seabird that I've seen before include "Gannet possibly" , "Gull, for example" , "A gull, for instance" , "Gull perhaps" , "gannet, perhaps" .)