Extremely anxious about church services (4)
I believe the answer is:
aces
'services' is the definition.
(ace is a type of serve in tennis)
'extremely anxious about church' is the wordplay.
'extremely' means to remove the middle letters.
'about' is an insertion indicator.
'church' becomes 'ce' (abbreviation for Church of England).
'anxious' with its centre taken out is 'as'.
'as' going around 'ce' is 'ACES'.
(Other definitions for aces that I've seen before include "that are non-returnable" , "Tennis serves the receiver can't reach" , "never come back" , "winning shots" , "passes with flying colours in US" .)