Possible Clues:

• Something as yet undeveloped -- more by rocks
• Occupant of a 52-Across
• First stage late, finally doctor on track with first of operations
• Number 10 defends British railway system that develops after congress
• Offspring in the early stages of development
• Early development phase spoken of by Mike with enthusiasm
• Mammal, before birth
• One starting to develop sacked by Rome
• Fertilized egg
• My robe is ragged, being in development