Join contrary moaners, every so often (4)
I believe the answer is:
seam
'join' is the definition.
(I know that seam is a more specific form of join)
'contrary moaners every so often' is the wordplay.
'contrary' says the letters should be written in reverse.
'every so often' indicates alternate letters.
The alternating letters of 'moaners' are 'maes'.
'maes' written backwards gives 'SEAM'.
(Other definitions for seam that I've seen before include "Join - stratum of ore" , "Underground layer of mineral" , "Stratum (of a mineral)" , "Fabric joint" , "Join - thin layer" .)