Do the normal clues first.
1a Medical advisors gutted laboratory/birds (6)
nice-ly arenaria
The 1st part of the 1a clue is the cryptic part of the definition
in clue 37a in an agreeable way.
The 2nd part of the 1a clue is the definition part of the cryptic
part in clue 37a About to engage in a new song. (a-re-n-aria)
Both of the answers nicely and arenaria are entered at other
locations than 1a or 37a.