Irritate girl and boy, losing head (5)
I believe the answer is:
annoy
'irritate' is the definition.
(I know that irritate can be written as annoy)
'girl and boy losing head' is the wordplay.
'girl' becomes 'noy' (I can't explain this - if you can you should give a lot more credence to this answer).
'and' means one lot of letters go next to another.
'boy' becomes 'ian' (man's name).
'losing head' means to remove the first letter.
'ian' with its first letter removed is 'an'.
'noy' put after 'an' is 'ANNOY'.
Can you help me to learn more?
(Other definitions for annoy that I've seen before include "trouble" , "get angry" , "Nark - nettle" , "Bug, rile" , "Irritate - pester" .)