Help Please With These Clues

[MarMatMal]

Difficulty with lower part of xword today. Help gratefully received.
18d Force unit to cover wide urban development (3,4) ???????
24a Force used when lifting is a problem (5,10) ????e/?????t????
15d Likely air to endless suffering (6,3) e?????/???
21d River always going in 2 directions (6) ??????
Many thanks in advance. Martin

[dannyk13]

21d.S ever n

[Toorak]

21 severn

[Lie-in King]

18d New town - 'Newton' (force unit, to cover) 'w(ide)'
24a Store detectives
15d Eldest son - anag 'to endless'
21d Severn - 'ever' (in) 's(outh) & n(orth)'

[Mamyalynne]

18 New Town - Newton round W(ise)

244 Store detectives

15 Eldest son
21 S ever N

[Toorak]

18 newto w n