Forceful and effective church, weak externally (6)
I believe the answer is:
punchy
'forceful and effective' is the definition.
'church weak externally' is the wordplay.
'church' becomes 'ch' (common abbreviation).
'weak' becomes 'puny' (similar in meaning).
'externally' is an insertion indicator (some letters go on the outside of others).
'ch' inserted inside 'puny' is 'PUNCHY'.
(Other definitions for punchy that I've seen before include "Incisive" , "Vigorous (in speech)" , "Sharp and powerful" , "Cutting" , "Hard-hitting" .)