Full back, put off, letting in goal (8)
I believe the answer is:
defender
'full back' is the definition.
'defender' can be an answer for 'back' (I've seen this before). I'm not sure about the 'full' bit.
'put off letting in goal' is the wordplay.
'put off' becomes 'defer' (synonyms).
'letting in' is an insertion indicator.
'goal' becomes 'end' (both can mean a purpose).
'defer' enclosing 'end' is 'DEFENDER'.
Can you help me to learn more?
(Other definitions for defender that I've seen before include "Back" , "One who protects from danger or takes one's side" , "Guardian" , "One who protects others" , "One resisting attack" .)