Reason the end of the string is in a tangled coil (5)
I believe the answer is:
logic
'reason' is the definition.
(I've seen this before)
'end of the string is in a tangled coil' is the wordplay.
'end of' suggests the final letters.
'is in' indicates putting letters inside.
'a tangled' is an anagram indicator.
The final letter of 'string' is 'g'.
'coil' anagrammed gives 'loic'.
'g' inserted into 'loic' is 'LOGIC'.
'the' acts as a link.
(Other definitions for logic that I've seen before include "Strict reasoning" , "Sound sense" , "good sense" , "method of reasoning" , "Reasonable judgement" .)