Lake swimmer in clip of Endeavour is revealed (5)
I believe the answer is:
Here is my best explanation:
'endeavour is revealed' is the definition.
I can't tell whether this defines the answer.
'lake swimmer in clip' is the subsidiary indication.
'lake swimmer' becomes 'u' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'in' means one lot of letters goes inside another.
'clip' becomes 'trot' ('trot' can be a synonym of 'clip').
'u' placed within 'trot' is 'trout'.
'of' is the link.
(Other definitions for trout that I've seen before include "Fish of salmon family", "Tasty fish, perhaps of rainbow kind", "Fish (associated with Schubert?)", "Mistiness", "Tasty fish, brown or rainbow", "air", "Rainbow, for example", "Prized river fish", "one of a noted quintet" and "one often tickled?".)