You'll find gymnastics initially tiring, dear (3)
I believe the answer is:
pet
'tiring dear' is the definition.
'pet' can be an answer for 'dear' (I've seen this in other clues). I am not certain of the 'tiring' bit.
'you'll find gymnastics initially' is the wordplay.
'you'll find' becomes 't' (I can't justify this - if you can you should give a lot more credence to this answer).
'gymnastics' becomes 'pe' (Physical Education).
'initially' means one lot of letters go next to another.
't' after 'pe' is 'PET'.
Can you help me to learn more?
(Other definitions for pet that I've seen before include "Cherished animal" , "Favourite, darling" , "Tamed animal; sulk" , "Cat or dog" , "The darling one in a small temper" .)