Pious girl oddly heartless (5)
I believe the answer is:
godly
'pious' is the definition.
(similar in meaning)
'girl oddly heartless' is the wordplay.
'girl' becomes 'g'.
'heartless' suggests removing the centre.
'oddly' with its middle removed is 'odly'.
'g'+'odly'='GODLY'
(Other definitions for godly that I've seen before include "Pious, decent" , "Reverential" , "Religious, pious" , "Divine" , "Devout" .)