Clue: Out in the open, bird keeps low – I'm delighted (4,3,4)
Answer: OVER THE MOON
Explanation: Out in the open, bird keeps low – I'm delighted (4,3,4)
**OVERT** ("out in the open") + **HEN** ("bird") *keeps* **MOO** ("low"). 
*Not sure that the "I"m" should be ther, other than to help the surface.