Trivial old bird in northern lake (7)
I believe the answer is:
nominal
'trivial' is the definition.
(thesaurus)
'old bird in northern lake' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'bird' becomes 'mina' (mina is a kind of bird**).
'in' means one lot of letters goes inside another.
'northern' becomes 'n' (abbreviation e.g. N. Ireland).
'lake' becomes 'l'.
'o'+'mina'='omina'
'n'+'l'='nl'
'omina' placed into 'nl' is 'NOMINAL'.
(Other definitions for nominal that I've seen before include "trifling" , "Relating to names" , "Token - lion man (anag)" , "Titular" , "Peppercorn rent" .)