Possible Clues:

• Defining characteristic
• Program variable
• Figure in a math function
• Soldier gathered with troops over boundary
• Short news item, an American measure – it may be the limit
• Limitation of safe containing a type of butter
• Is your old man an eater of mutton? That's about the size of it
• Factor defining limits
• For a sheepish creature in safe environs this is the limiting factor
• Father, the artist, came across 'er - that's the limit