Retro science facility, source of medicinal lotion (4)
I believe the answer is:
balm
'lotion' is the definition.
(I've seen this in another clue)
'retro science facility source of medicinal' is the wordplay.
'retro' shows that the letters should be reversed in order (retro is Latin for 'backwards').
'science facility' becomes 'lab' (laboratory).
'source of' indicates taking the first letters.
The initial letter of 'medicinal' is 'm'.
'lab' written backwards gives 'bal'.
'bal'+'m'='BALM'
(Other definitions for balm that I've seen before include "Unguent" , "It's comforting" , "It comforts, soothes" , "Soothing herb" , "Soothing ointment for lamb (4)" .)