I think the preamble was mentioned on another EV thread.
Basically compass point letters can be missing form clues....and some answers aren't as long as the space in the grid
3d Angler's lie: no source of fish in historic water (7) MO???I? possibly angler's liNe?
19d Old fashioned French age upset about heartless addition to contract (7) A????R?
20d Article on investigation overlooking Southern heaths (7) possibly Sheaths?
Yes N for 3d - Angler's li[N]e: no source of fish in historic water = MONOFIL 'moil' = 'water' (archaic)
19d (I see you've removed part of the phrase) - Old fashioned French age upset about heartless addition to contract = ARRIERE 'age' = ERA (rev) around 'ri[d]er'
Yes S for 20d - Article on investigation overlooking Southern [S]heaths THECAE 'ca(s)e' = 'investigation'.
15d, [W]Hal[E]s etc at work- excellent art at the outset - CETACEA [anag 'etc' + ACE + A(rt)].
24a, Wh[E]at relative runs colliding with you once - RYE [R(uns) + YE].