Turning the Spanish girl is permissible (5)
I believe the answer is:
legal
'permissible' is the definition.
(I've seen this before)
'turning the spanish girl' is the wordplay.
'turning' is a reversal indicator.
'the spanish' becomes 'la' ('the' in Spanish).
'girl' becomes 'gel' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'la'+'gel'='lagel'
'lagel' back-to-front is 'LEGAL'.
'is' acts as a link.
Can you help me to learn more?
(Other definitions for legal that I've seen before include "Allowed by law" , "It's acceptable" , "Street - (1978 Dylan album)" , "In accordance with law" , "Licit" .)