Love to escape from unfastened enclosure (3)
I believe the answer is:
pen
'enclosure' is the definition.
(pen is a kind of enclosure)
'love to escape from unfastened' is the wordplay.
'love' becomes 'o' (love means zero in tennis).
'to escape from' suggests deleting specific letters.
'unfastened' becomes 'open' (similar in meaning).
'open' with 'o' taken away is 'PEN'.
(Other definitions for pen that I've seen before include "Pound" , "American jug?" , "'The ... is mightier than the sword'" , "Prison" , "dash off" .)