Girl that's keeping close to zoo keeper (6)
I believe the answer is:
goalie
'keeper' is the definition.
(I've seen this before)
'girl that's keeping close to zoo' is the wordplay.
'girl' becomes 'gal' ('gal' can be a synonym of 'girl'**).
'that's' becomes 'ie'.
'keeping' indicates putting letters inside.
'close to' indicates one should take the final letters (the close/ending of).
The final letter of 'zoo' is 'o'.
'gal'+'ie'='galie'
'galie' enclosing 'o' is 'GOALIE'.
(Other definitions for goalie that I've seen before include "Soccer player who protects the target" , "On-field custodian" , "Keeper (colloq.)" , "Football team member" , "one saves" .)