First love in a girl is moving (8)
I believe the answer is:
original
'first' is the definition.
('original' is associated in meaning with 'first')
'love in a girl is moving' is the wordplay.
'love' becomes 'o' (In tennis, 'love' means 'zero').
'is moving' is an anagram indicator.
'in'+'a'+'girl'='inagirl'
'inagirl' with letters rearranged gives 'riginal'.
'o'+'riginal'='ORIGINAL'
(Other definitions for original that I've seen before include "New in character or design - not a copy" , "Never seen or performed before" , "Existing from the start - quite new" , "From which a copy is made" , "Novel - archetype" .)