Trivial old bird in northern lake (7)
I believe the answer is:
Here is my best explanation:
'trivial' is the definition.
'old bird in northern lake' is the subsidiary indication.
'old' becomes 'o'.
'bird' becomes 'mina' (mina is a kind of bird**).
'in' is an insertion indicator.
'northern' becomes 'n' (abbreviation e.g. N. Ireland).
'lake' becomes 'l'.
'omina' going into 'nl' is 'nominal'.
(Other definitions for nominal that I've seen before include "In name only or of token value (7)", "at such a low price?", "Minor", "Existing in name only", "Relating to names", "Of token value", "Titular", "Only in name", "In name only or token payment" and "Token - lion-man (anag)".)