She's a stick-in-the-mud (4)
I believe the answer is:
maud
'she's' is the definition.
'a stick-in-the-mud' is the wordplay.
I cannot really see how this works, but
'a' is found in the answer.
'the mud' could be 'mud' and 'mud' is present in the remaining letters.
No letters remain.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for maud that I've seen before include "Tennyson poem" , "Come into the garden, - (Tennyson)" , "Old song: ''Come into the garden . . . .''" , "Girl invited into the garden in song" , "Girl invited into garden (Tennyson)" .)