### Clue: in favour spot object in 21 having very strong 20 (5,4)
### Answer: BLACK HOLE 
 ### Explanation: Definition: object in 21 having very strong 20
  L(Roman numeral for 50) **contained in**(in) BACK(to favour,support) + HOLE(a dangerous or difficult position,spot, as in "I found myself in a spot/hole").
Answer:  An object in space(answer to 21across) having very strong gravity(answer to 20down), so strong that not even light escapes it.