Bird dog might show pleasure thus (7)
I believe the answer is:
wagtail
'bird' is the definition.
(wagtail is a kind of bird)
'dog might show pleasure thus' is the wordplay.
I cannot quite understand how this works, but
'dog' could be 'tail' (synonyms) and 'tail' is located in the answer.
The remaining letters 'wag' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for wagtail that I've seen before include "Songbird - gala wit (anag)" , "singer" , "I tag law of the bird" , "Common bird" , "'Bird, its posterior projection jerks up and down (7)'" .)