Cotton on a reel is tangled (7)
I believe the answer is:
realise
'cotton on' is the definition.
(I've seen this before)
'a reel is tangled' is the wordplay.
'tangled' indicates anagramming the letters.
'a'+'reel'+'is'='areelis'
'areelis' is an anagram of 'REALISE'.
(Other definitions for realise that I've seen before include "make a profit" , "Accomplish; see rail (anag.)" , "Ears lie (anag.)" , "Bring to fruition" , "get cash for property" .)