It's used to fix useless tap, leaking internally (6)
I believe the answer is:
staple
'it's used' is the definition.
I can't tell whether this definition defines the answer.
'fix useless tap leaking internally' is the wordplay.
'internally' indicates the answer is hidden within the clue.
'STAPLE' can be found hidden inside 'fix useless tap leaking'.
'to' is the link.
Can you help me to learn more?
(Other definitions for staple that I've seen before include "It holds fast" , "important things to eat" , "e.g. rice, sweetcorn ..." , "Paper pin for plates" , "Important; paper fastener" .)