Old man who might have long legs? (5)
I believe the answer is:
daddy
'old man who might have long legs?' is the definition.
'daddy' can be an answer for 'old man' (synonyms). I am not certain of the remainder of the definition.
'old man who might' is the wordplay.
'old man' becomes 'dad' (both can mean one's father).
'who might' becomes 'dy' (I can't justify this - if you can you should give a lot more credence to this answer).
'dad'+'dy'='DADDY'
Can you help me to learn more?
(Other definitions for daddy that I've seen before include "Informal father" , "Pa" , ">one of the Fathers" , "pop" , "Papa" .)